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Why are we able to differentiate between colored objects without the presence of light?

Why are we able to differentiate between colored objects without the presence of light?


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Rods help us to see during the dark.

We can see colour of the object when it reflects light.

But in a dark room(room of any color but the light is switched off and it is night),we still can differentiate between colored objects if we open our eyes for sometime.(although not very much clearly)

How the colors are seen when there is no light?

Do we see as the brain remembers their colour?


You seem to have many misconceptions about how we see and how that relates to light. I'll address them one sentence at a time:

Why are we able to differentiate between colored objects without the presence of light?

Answer: the premise of the question is wrong: we aren't able to differentiate between colors without the presence of light, because we aren't able to see without the presence of light. Light is what allows us to see. "Seeing" is the act of detecting the light coming from the environment and using it to derive what the environment is like.

Rods help us to see during the dark.

It sounds a bit like you think rods help us to see during the dark… without light ? This is not the case: BOTH rods and cones absorb light and thus allow us to see. Rods help us see in low light because they are more sensitive (to light !) than cones. Nothing can help us see with no light.

We can see colour of the object when it reflects light.

It depends on how much light it reflects ! The correct form of that sentence is "We can see the object when it reflects light". (even more correct would be "when it reflects enough light to trigger the light receptors in our eyes, i.e. rods and cones). Whether we can see its colour depends on whether it reflects enough light to trigger the cones specifically.

But in a dark room(room of any color but the light is switched off and it is night),we still can differentiate between colored objects if we open our eyes for sometime.(although not very much clearly)

Indeed; this is because no room is completely dark. Even more so in our artificially lit world, but even in a moonless cloudy night you may have a few errant photons coming from the stars and Sun (via diffusion in the atmosphere), fireflies, what have you; to be fair in our artificially lit world I haven't experienced it. As far as I know you can get true pitch darkness in things like underground caves (once you're far enough from an exit of course).

Basically, if you can see anything then there is some light. (well, short of your optical cortex confabulating stuff, which in fairness it will do).

How the colors are seen when there is no light?

As I hope you understand now, they aren't.

Do we see as the brain remembers their colour?

I think that may actually be one reason we perceive colour in low-light environments. I'm sure some of it is that there's enough light to trigger the cones a little bit, but I wouldn't underestimate our brain's ability to show us the world as it knows it is as opposed to as whatever information it's receiving right this second says it looks like.

EDIT

Some references :

The state of seeing things in such low light that we only use the rods is called "scotopic vision" :
https://en.wikipedia.org/wiki/Scotopic_vision

Some sources for how "dark" environments still contain low amounts of light (one word for "amount of light" being "illuminance", measured in Lux) :
http://www.engineeringtoolbox.com/light-level-rooms-d_708.html
https://en.wikipedia.org/wiki/Aphotic_zone (thank you to @Johnny for that one)

HOPEFULLY LAST EDIT

On reflection I thought I'd say some more about scotopic vision since it's very relevant to the question. It turns out "scotopic" vision is opposed to "photopic" vision (the vision we have in full lighting, where the cones do the work because the rods are saturated), and in between there is an intermediate range called "mesopic" vision, which is likely the kind of vision we experience when it's dark but we still see color (this is ignoring our brain filling in colors, which could also be a factor and in that case could also happen with scotopic vision - but I didn't find any sources on that).

Mesopic vision is used under lighting conditions that seem to range from low indoor lighting or streetlights to starlight; scotopic vision is used with luminances darker than that. This page is all about it and includes a very nice drawing, sadly it's a pdf so I can't show it in the answer:

http://www.visual-3d.com/Education/LightingLessons/Documents/PhotopicScotopiclumens_4%20_2_.pdf


Human Vision and Color Perception

Human stereo color vision is a very complex process that is not completely understood, despite hundreds of years of intense study and modeling. Vision involves the nearly simultaneous interaction of the two eyes and the brain through a network of neurons, receptors, and other specialized cells. The first steps in this sensory process are the stimulation of light receptors in the eyes, conversion of the light stimuli or images into signals, and transmission of electrical signals containing the vision information from each eye to the brain through the optic nerves. This information is processed in several stages, ultimately reaching the visual cortices of the cerebrum.

The human eye is equipped with a variety of optical components including the cornea, iris, pupil, aqueous and vitreous humors, a variable-focus lens, and the retina (as illustrated in Figure 1). Together, these elements work to form images of the objects that fall into the field of view for each eye. When an object is observed, it is first focused through the convex cornea and lens elements, forming an inverted image on the surface of the retina, a multi-layered membrane that contains millions of light-sensitive cells. In order to reach the retina, light rays focused by the cornea must successively traverse the aqueous humor (in the anterior chamber), the crystalline lens, the gelatinous vitreous body, and the vascular and neuronal layers of the retina before they reach the photosensitive outer segments of the cone and rod cells. These photosensory cells detect the image and translate it into a series of electrical signals for transmission to the brain.

Despite some misconceptions due to the wide spectrum of terminology employed for describing eye anatomy, it is the cornea, not the lens, which is responsible for the major part of the total refractive power of the eye. Being smooth and clear as glass, yet as flexible and durable as plastic, the anterior, strongly curved, transparent part of the exterior wall of the eyeball allows the image-forming light rays to pass through to the interior. The cornea also protects the eye by providing a physical barrier that shields the inside of the eye from microorganisms, dust, fibers, chemical, and other harmful materials. Although much thinner in width than the crystalline lens, the cornea provides about 65 percent of the eye's refractive power. Most of the power to bend light resides near the center of the cornea, which is rounder and thinner than the peripheral portions of the tissue.

As the window that controls the entry of light into the eye, the cornea (Figure 2) is essential to good vision and also acts as an ultraviolet light filter. The cornea removes some of the most damaging ultraviolet wavelengths present in sunlight, thereby further protecting the highly susceptible retina and crystalline lens from damage. If the cornea is curved too much, as in the case of nearsightedness, distant objects will appear as blurry images, because of imperfect light refraction to the retina. In a condition known as astigmatism, imperfections or irregularities in the cornea result in unequal refraction, which creates distortion of images projected onto the retina.

Unlike most tissues of the body, the cornea does not contain blood vessels for nourishment or to protect it against infection. Even the smallest capillaries would interfere with the precise refraction process. The cornea receives its nourishment from tears and the aqueous humor, which fills the chambers behind the structure. The outer epithelial layer of the cornea is packed with thousands of small nerve endings, making the cornea extremely sensitive to pain when rubbed or scratched. Comprising about 10 percent of the tissue's thickness, the epithelial layer of the cornea blocks foreign matter from entering the eye while providing a smooth surface for oxygen and nutrient absorption. The central layer of the cornea, known as the stroma, comprises about 90 percent of the tissue, and consists of a water-saturated fibrous protein network that provides strength, elasticity, and form to support the epithelium. Nourishing cells complete the remainder of the stroma layer. Because the stroma tends to absorb water, the endothelium tissue's primary task is to pump excess water from the stroma. Without this pumping action, the stroma would swell with water, become hazy, and ultimately turn the cornea opaque, rendering the eye blind.

The partial or complete loss of transparency by the crystalline lens, or its capsule, results in a common condition known as cataracts. Cataracts are the leading cause of blindness worldwide and represent an important cause of visual impairment in the United States. Development of cataracts in adults is related to normal aging, sunlight exposure, smoking, poor nutrition, eye trauma, systemic disease such as diabetes and glaucoma, and undesirable side effects from some pharmaceuticals, including steroids. In the early stages, an individual suffering from cataracts perceives the world as blurry or out of focus. Clear vision is prevented by a reduction in the amount of light that reaches the retina and by clouding of the image (through diffraction and light scattering) as though the individual were observing the environment through a fog or haze (see Figure 3). Removal of the opaque lens during cataract surgery, with subsequent replacement by a plastic lens (intraocular lens implants), often results in corrected vision for unrelated conditions such as nearsightedness or farsightedness.

The function of the retina is similar to the combination of a digital image sensor (such as a charge-coupled device (CCD)) with an analog-to-digital converter, as featured in modern digital camera systems. The image-capturing receptors of the eyes, known as rods and cones, are connected with the fibers of the optic nerve bundle through a series of specialized cells that coordinate the transmission of signals to the brain. The amount of light allowed to enter each eye is controlled by the iris, a circular diaphragm that opens wide at low light levels and closes to protect the pupil (the aperture) and retina at very high levels of illumination.

As illumination changes, the diameter of the pupil (positioned in front of the crystalline lens) reflexively varies between a size of about 2 to 8 millimeters, modulating the amount of light that reaches the retina. When illumination is very bright, the pupil narrows and peripheral portions of the refractile elements are excluded from the optical pathway. The result is that fewer aberrations are encountered by image-forming light rays, and the image on the retina becomes sharper. A very narrow pupil (approximately 2 millimeters) produces diffraction artifacts that spread the image of a point source on the retina.

In the brain, the neural fibers of the optic nerves from each eye cross at the optic chiasma where visual information from both retinas traveling in parallel pathways is correlated, somewhat like the function of a time base correction generator in a digital video tape recorder. From there, the visual information travels through the optic tract to the knee-shaped lateral geniculate nuclei in the thalamus, where the signals are distributed via the optic radiations to the two visual cortices located on the lower rear section of each half of the cerebrum. In the lower layers of the cortex, the information from each eye is maintained as columnar ocular dominance stripes. As the visual signals are transmitted to the upper layers of the cortex, information from the two eyes is merged and binocular vision is formed. In abnormal ophthalmic conditions such as phorias (misalignments) of the eyes, including strabismus (better known as crossed-eyes), stereovision is disrupted as are the individual's bearings and depth perception. In cases where ophthalmic surgery is not warranted, prismatic lenses mounted in spectacles can correct some of these anomalies. Causes of interruption to the binocular fusion may be head or birth trauma, neuromuscular disease, or congenital defects.

The central fovea is located in an area near the center of the retina, and positioned directly along the optical axis of each eye. Known also as the "yellow spot", the fovea is small (less than 1 square millimeter), but very specialized. These areas contain exclusively high-density, tightly packed cone cells (greater than 200,000 cones per square millimeter in adult humans see Figure 4). The central fovea is the area of sharpest vision, and produces the maximum resolution of space (spatial resolution), contrast, and color. Each eye is populated with approximately seven million cone cells, which are very thin (3 micrometers in diameter) and elongated. The density of cone cells decreases outside of the fovea as the ratio of rod cells to cone cells gradually increases (Figure 4). At the periphery of the retina, the total number of both types of light receptors decreases substantially, causing a dramatic loss of visual sensitivity at the retinal borders. This is offset by the fact that humans constantly scan objects in the field of view (due to involuntary rapid eye movements), resulting in a perceived image that is uniformly sharp. In fact, when the image is prevented from moving relative to the retina (via an optical fixation device), the eye no longer senses an image after a few seconds.

The arrangement of sensory receptors in the outer segments of the retina partially determine the limit of resolution in different regions of the eye. In order to resolve an image, a row of less-stimulated photoreceptors must be interposed between two rows of photoreceptors that are highly stimulated. Otherwise, it is impossible to distinguish whether the stimulation originated from two closely spaced images or from a single image that spans the two receptor rows. With a center-to-center spacing ranging between 1.5 and 2 micrometers for the cones in the central fovea, optical stimuli having a separation of approximately 3 to 4 micrometers should produce a resolvable set of intensities on the retina. For reference, the radius of the first minimum for a diffraction pattern formed on the retina is about 4.6 micrometers with 550-nanometer light and a pupil diameter of 2 millimeters. Thus, the arrangement of sensory elements in the retina will determine the limiting resolution of the eye. Another factor, termed visual acuity (the ability of the eye to detect small objects and resolve their separation), varies with many parameters, including the definition of the term and the method by which acuity is measured. Over the retina, visual acuity is generally highest in the central fovea, which spans a visual field of about 1.4 degrees.

The spatial arrangement of rod and cone cells and their connection to neurons within the retina is presented in Figure 5. Rod cells, containing only the photopigment rhodopsin, have a peak sensitivity to blue-green light (wavelength of about 500 nanometers), although they display a broad range of response throughout the visible spectrum. They are the most common visual receptor cells, with each eye containing about 125-130 million rod cells. The light sensitivity of rod cells is about 1,000 times that of cone cells. However, the images generated by rod stimulation alone are relatively unsharp and confined to shades of gray, similar to those found in a black and white soft-focus photographic image. Rod vision is commonly referred to as scotopic or twilight vision because in low light conditions, shapes and the relative brightness of objects can be distinguished, but not their colors. This mechanism of dark adaptation enables the detection of potential prey and predators via shape and motion in a wide spectrum of vertebrates.

The human visual system response is logarithmic, not linear, resulting in the ability to perceive an incredible brightness range (interscene dynamic range) of over 10 decades. In broad daylight, humans can visualize objects in the glaring light from the sun, while at night large objects can be detected by starlight when the moon is dark. At threshold sensitivity, the human eye can detect the presence of about 100-150 photons of blue-green light (500 nanometers) entering the pupil. For the upper seven decades of brightness, photopic vision predominates, and it is the retinal cones that are primarily responsible for photoreception. In contrast, the lower four decades of brightness, termed scotopic vision, are controlled by the rod cells.

Adaptation of the eye enables vision to function under such extremes of brightness. However, during the interval of time before adaptation occurs, individuals can sense a range of brightness covering only about three decades. Several mechanisms are responsible for the ability of the eye to adapt to a high range of brightness levels. Adaptation can occur in seconds (by initial pupillary reaction) or may take several minutes (for dark adaptation), depending upon the level of brightness change. Full cone sensitivity is reached in about 5 minutes, whereas it requires about 30 minutes to adapt from moderate photopic sensitivity to the full scoptic sensitivity produced by the rod cells.

When fully light-adapted, the human eye features a wavelength response from around 400 to 700 nanometers, with a peak sensitivity at 555 nanometers (in the green region of the visible light spectrum). The dark-adapted eye responds to a lower range of wavelengths between 380 and 650 nanometers, with the peak occurring at 507 nanometers. For both photopic and scoptic vision, these wavelengths are not absolute, but vary with the intensity of light. The transmission of light through the eye becomes progressively lower at shorter wavelengths. In the blue-green region (500 nanometers), only about 50 percent of light entering the eye reaches the image point on the retina. At 400 nanometers, this value is reduced to a scant 10 percent, even in a young eye. Light scattering and absorption by elements in the crystalline lens contributes to a further loss of sensitivity in the far blue.

Cones consist of three cell types, each "tuned" to a distinct wavelength response maximum centered at either 430, 535, or 590 nanometers. The basis for the individual maxima is the utilization of three different photopigments, each with a characteristic visible light absorption spectrum. The photopigments alter their conformation when a photon is detected, enabling them to react with transducin to initiate a cascade of visual events. Transducin is a protein that resides in the retina and is able to effectively convert light energy into an electrical signal. The population of cone cells is much smaller than rod cells, with each eye containing between 5 and 7 million of these color receptors. True color vision is induced by the stimulation of cone cells. The relative intensity and wavelength distribution of light impacting on each of the three cone receptor types determines the color that is imaged (as a mosaic), in a manner comparable to an additive RGB video monitor or CCD color camera.

A beam of light that contains mostly short-wavelength blue radiation stimulates the cone cells that respond to 430-nanometer light to a far greater extent than the other two cone types. This beam will activate the blue color pigment in specific cones, and that light is perceived as blue. Light with a majority of wavelengths centered around 550 nanometers is seen as green, and a beam containing mostly 600 nanometer wavelengths or longer is visualized as red. As mentioned above, pure cone vision is referred to as photopic vision and is dominant at normal light levels, both indoors and out. Most mammals are dichromats, usually able to only distinguish between bluish and greenish color components. In contrast, some primates (most notably humans) exhibit trichromatic color vision, with significant response to red, green and blue light stimuli.

Illustrated in Figure 6 are the absorption spectra of the four human visual pigments, which display maxima in the expected red, green, and blue regions of the visible light spectrum. When all three types of cone cell are stimulated equally, the light is perceived as being achromatic or white. For example, noon sunlight appears as white light to humans, because it contains approximately equal amounts of red, green, and blue light. An excellent demonstration of the color spectrum from sunlight is the interception of the light by a glass prism, which refracts (or bends) different wavelengths to varying degrees, spreading out the light into its component colors. Human color perception is dependent upon the interaction of all receptor cells with light, and this combination results in nearly trichromic stimulation. There are shifts in color sensitivity with variations in light levels, so that blue colors look relatively brighter in dim light and red colors look brighter in bright light. This effect can be observed by pointing a flashlight onto a color print, which will result in the reds suddenly appearing much brighter and more saturated.

In recent years, consideration of human color visual sensitivity has led to changes in the long-standing practice of painting emergency vehicles, such as fire trucks and ambulances, entirely red. Although the color is intended for the vehicles to be easily seen and responded to, the wavelength distribution is not highly visible at low light levels and appears nearly black at night. The human eye is much more sensitive to yellow-green or similar hues, particularly at night, and now most new emergency vehicles are at least partially painted a vivid yellowish green or white, often retaining some red highlights in the interest of tradition.

When only one or two types of cone cells are stimulated, the range of perceived colors is limited. For example, if a narrow band of green light (540 to 550 nanometers) is used to stimulate all of the cone cells, only the ones containing green photoreceptors will respond to produce a sensation of seeing the color green. Human visual perception of primary subtractive colors, such as yellow, can arise in one of two ways. If the red and green cone cells are simultaneously stimulated with monochromatic yellow light having a wavelength of 580 nanometers, the cone cell receptors each respond almost equally because their absorption spectral overlap is approximately the same in this region of the visible light spectrum. The same color sensation can be achieved by stimulating the red and green cone cells individually with a mixture of distinct red and green wavelengths selected from regions of the receptor absorption spectra that do not have significant overlap. The result, in both cases, is simultaneous stimulation of red and green cone cells to produce a sensation of yellow color, even though the end result is achieved by two different mechanisms. The ability to perceive other colors requires the stimulation of one, two, or all three types of cone cells, to various degrees, with the appropriate wavelength palette.

Although the human visual system features three types of cones cells with their respective color pigments plus light-receptive rod cells for scotopic vision, it is the human brain that compensates for variations of light wavelengths and light sources in its perception of color. Metamers are pairs of different light spectra perceived as the same color by the human brain. Interestingly, colors that are interpreted as the same or similar by a human are sometimes readily distinguishable by other animals, most notably birds.

Intermediary neurons that ferry visual information between the retina and the brain are not simply connected one-to-one with the sensory cells. Each cone and rod cell in the fovea sends signals to at least three bipolar cells, whereas in the more peripheral regions of the retina, signals from large numbers of rod cells converge to a single ganglion cell. Spatial resolution in the outer portions of the retina is compromised by having a large number of rod cells feeding a single channel, but having many sensory cells participate in capturing weak signals significantly improves the threshold sensitivity of the eye. This feature of the human eye is somewhat analogous to the consequence of binning in slow-scan CCD digital camera systems.

The sensory, bipolar cells, and ganglion cells of the retina are also interconnected to other neurons, providing a complex network of inhibitory and excitatory pathways. As a result, the signals from the 5 to 7 million cones and 125 million rods in the human retina are processed and transported to the visual cortex by only about 1 million myelinated optical nerve fibers. The eye muscles are stimulated and controlled by ganglion cells in the lateral geniculate body, which acts as a feedback control between the retina and the visual cortex.

The complex network of excitatory and inhibitory pathways in the retina are arranged in three layers of neuronal cells that arise from a specific region of the brain during embryonic development. These circuits and feedback loops result in a combination of effects that produce edge sharpening, contrast enhancement, spatial summation, noise averaging, and other forms of signal processing, perhaps including some that have not yet been discovered. In human vision, a significant degree of image processing takes place in the brain, but the retina itself also is involved in a wide range of processing tasks.

In another aspect of human vision known as color invariance, the color or gray value of an object does not appear to change over a wide range of luminance. In 1672, Sir Isaac Newton demonstrated color invariance in human visual sensation and provided clues for the classical theory of color perception and the nervous system. Edwin H. Land, founder of the Polaroid Corporation, proposed the Retinex theory of color vision, based on his observations of color invariance. As long as color (or a gray value) is viewed under adequate lighting, a color patch does not change its color even when the luminance of the scene is changed. In this case, a gradient of illumination across the scene does not alter the perceived color or gray-level tone of a patch. If the luminance level reaches the threshold for scotopic or twilight vision, the sensation of color vanishes. In Land's algorithm, the lightness values of colored areas are computed, and the energy at a particular area in the scene is compared with all the other areas in the scene for that waveband. The calculations are performed three times, one for each waveband (long wave, short wave, and middle wave), and the resulting triplet of lightness values determines a position for the area in the three-dimensional color space defined by the Retinex theory.

The term color blindness is something of a misnomer, being widely used in colloquial conversation to refer to any difficulty in distinguishing between colors. True color blindness, or the inability to see any color, is extremely rare, although as many as 8 percent of men and 0.5 percent of women are born with some form of color vision defect (see Table 1). Inherited deficiencies in color vision are usually the result of defects in the photoreceptor cells in the retina, a neuro-membrane that functions as the imaging surface at the rear of the eye. Color vision defects can also be acquired, as a result of disease, side effects of certain medications, or through normal aging processes, and these deficiencies may affect parts of the eye other than the photoreceptors.

Normal cones and pigment sensitivity enable an individual to distinguish all the different colors as well as subtle mixtures of hues. This type of normal color vision is known as trichromacy and relies upon the mutual interaction from the overlapping sensitivity ranges of all three types of photoreceptor cone. A mild color vision deficiency occurs when the pigment in one of the three cone types has a defect, and its peak sensitivity is shifted to another wavelength, producing a visual deficiency termed anomalous trichromacy, one of three broad categories of color vision defect. Dichromacy, a more severe form of color blindness, or color deficiency, occurs when one of the pigments is seriously deviant in its absorption characteristics, or the particular pigment has not been produced at all. The complete absence of color sensation, or monochromacy, is extremely rare, but individuals with total color blindness (rod monochromats) see only varying degrees of brightness, and the world appears in black, white, and shades of gray. This condition occurs only in individuals who inherit a gene for the disorder from both parents.

Dichromats can distinguish some colors, and are therefore less affected in their daily lives than monochromats, but they are usually aware that they have a problem with their color vision. Dichromacy is subdivided into three types: protanopia, deuteranopia, and tritanopia (see Figure 7). Approximately two percent of the male population inherits one of the first two types, with the third occurring much more rarely.

Ishihara Color Blindness Test

Color blindness, a disruption in the normal functioning of human photopic vision, can be caused by host of conditions, including those derived from genetics, biochemistry, physical damage, and diseases. This interactive tutorial explores and simulates how full-color images appear to colorblind individuals, and compares these images to the Ishihara diagnostic colorblind test.

Protanopia is a red-green defect, resulting from loss of red sensitivity, which causes a lack of perceptible difference between red, orange, yellow, and green. In addition, the brightness of red, orange, and yellow colors is dramatically reduced in comparison to normal levels. The reduced intensity effect can result in red traffic lights appearing dark (unlit), and red hues (in general), appearing as black or dark gray. Protanopes often learn to correctly distinguish between red and green, and red from yellow, primarily based on their apparent brightness, rather than on any perceptible hue difference. Green generally appears lighter than red to these individuals. Because red light occurs at one end of the visible spectrum, there is little overlap in sensitivity with the other two cone types, and people with protanopia have a pronounced loss of sensitivity to light at the long-wavelength (red) end of the spectrum. Individuals with this color vision defect can discriminate between blues and yellows, but lavender, violet, and purple cannot be distinguished from various shades of blue, due to the attenuation of the red component in these hues.

Individuals with deuteranopia, which is a loss of green sensitivity, have many of the same problems with hue discrimination as do protanopes, but have a fairly normal level of sensitivity across the visible spectrum. Because of the location of green light in the center of the visible light spectrum, and the overlapping sensitivity curves of the cone receptors, there is some response of the red and blue photoreceptors to green wavelengths. Although deuteranopia is associated with at least a brightness response to green light (and little abnormal intensity reduction), the names red, orange, yellow, and green seem to the deuteranope to be too many terms for colors that appear the same. In a similar fashion, blues, violets, purples, and lavenders are not distinguishable to individuals with this color vision defect.

Color Blindness Incidence and Causes
CLASSIFICATIONCAUSE OF DEFECTINCIDENCE
(%)
Anomalous Trichromacy 6.0
ProtanomalyAbnormal Red-Sensing Pigment1.0
DeuteranomalyAbnormal Green-Sensing Pigment5.0
TritanomalyAbnormal Blue-Sensing Pigment0.0001
Dichromacy 2.1
ProtanopiaAbsent Red-Sensing Pigment1.0
DeuteranopiaAbsent Green-Sensing Pigment1.1
TritanopiaAbsent Blue-Sensing Pigment0.001
Rod MonochromacyNo Functioning Cones< 0.0001
Table 1

Tritanopia is the absence of blue sensitivity, and functionally produces a blue-yellow defect in color vision. Individuals with this deficiency cannot distinguish blues and yellows, but do register a difference between red and green. The condition is quite rare, and occurs about equally in both sexes. Tritanopes usually do not have as much difficulty in performing everyday tasks as do individuals with either of the red-green variants of dichromacy. Because blue wavelengths occur only at one end of the spectrum, and there is little overlap in sensitivity with the other two cone types, total loss of sensitivity across the spectrum can be quite severe with this condition.

When there is a loss of sensitivity by a cone receptor, but the cones are still functional, resulting color vision deficiencies are considered anomalous trichromacy, and they are categorized in a similar manner to the dichromacy types. Confusion often arises because these conditions are named similarly, but appended with a suffix derived from the term anomaly. Thus, protanomaly, and deuteranomaly produce hue recognition problems that are similar to the red-green dichromacy defects, though not as pronounced. Protanomaly is considered a "red weakness" of color vision, with red (or any color having a red component) being visualized as lighter than normal, and hues shifted toward green. A deuteranomalous individual exhibits "green weakness", and has similar difficulties in discriminating between small variations in hues falling in the red, orange, yellow, and green region of the visible spectrum. This occurs because the hues appear to be shifted toward red. In contrast, deuteranomalous individuals do not have the brightness loss defect that accompanies protanomaly. Many people with these anomalous trichromacy variants have little difficulty performing tasks that require normal color vision, and some may not even be aware that their color vision is impaired. Tritanomaly, or blue weakness, has not been reported as an inherited defect. In the few cases in which the deficiency has been identified, it is thought to have been acquired rather than inherited. Several eye diseases (such as glaucoma, which attacks the blue cones) can result in tritanomaly. Peripheral blue cone loss is most common in these diseases.

In spite of the limitations, there are some visual acuity advantages to color blindness, such as the increased ability to discriminate camouflaged objects. Outlines, rather than colors, are responsible for pattern recognition, and improvements in night vision may occur due to certain color vision deficiencies. In the military, colorblind snipers and spotters are highly valued for these reasons. During the early 1900s, in an effort to evaluate abnormal human color vision, the Nagel anomaloscope was developed. Utilizing this instrument, the observer manipulates control knobs to match two colored fields for color and brightness. Another evaluation method, the Ishihara pseudoisochromatic plate test for color blindness, named for Dr. Shinobu Ishihara, discriminates between normal color vision and red-green color blindness (as presented in the tutorial and Figure 7). A test subject with normal color vision can detect the hue difference between the figure and background. To an observer with red-green deficiency, the plates appear isochromatic with no discrimination between the figures and the design pattern.

As a natural part of the aging process, the human eye begins to perceive colors differently in later years, but does not become "colorblind" in the true sense of the term. Aging results in the yellowing and darkening of the crystalline lens and cornea, degenerative effects that are also accompanied by a shrinking of the pupil size. With yellowing, shorter wavelengths of visible light are absorbed, so blue hues appear darker. As a consequence, elderly individuals often experience difficulty discriminating between colors that differ primarily in their blue content, such as blue and gray or red and purple. At age 60, when compared to the visual efficiency of a 20-year old, only 33 percent of the light incident on the cornea reaches the photoreceptors in the retina. This value drops to around 12.5 percent by the mid-70s.

Human Eye Accommodation

Accommodation of the eye refers to the physiological act of adjusting crystalline lens elements to alter the refractive power and bring objects that are closer to the eye into sharp focus. This tutorial explores changes in the lens structure as objects are relocated with respect to the eye.

Accommodation of the eye refers to the act of physiologically adjusting the crystalline lens element to alter the refractive power and bring objects that are closer to the eye into sharp focus. Light rays initially refracted at the surface of the cornea are further converged after passing through the lens. During accommodation, contraction of the ciliary muscles relaxes tension on the lens, resulting in changes to the shape of the transparent and elastic tissue, while also moving it slightly forward. The net effect of the lens alterations is to adjust the focal length of the eye to bring the image exactly into focus onto the photosensitive layer of cells residing in the retina. Accommodation also relaxes the tension applied to the lens by the zonule fibers, and allows the anterior surface of the lens to increase its curvature. The increased degree of refraction, coupled with a slight forward shift in the position of the lens, brings objects that are closer to the eye into focus.

Focus in the eye is controlled by a combination of elements including the iris, lens, cornea, and muscle tissue, which can alter the shape of the lens so the eye can focus on both nearby and distant objects. However, in some instances these muscles do not work properly or the eye is slightly altered in shape, and the focal point does not intersect with the retina (a condition termed convergent vision). As individuals age, the lens becomes harder and cannot be properly focused, leading to poor vision. If the point of focus falls short of the retina, the condition is referred to as nearsightedness or myopia, and individuals with this affliction cannot focus on distant objects. In cases where the focal point is behind the retina, the eye will have trouble focusing on nearby objects, creating a condition known as farsightedness or hypermetropia. These malfunctions of the eye can usually be corrected with eyeglasses (Figure 8) using a concave lens to treat myopia and a convex lens to treat hypermetropia.

Convergent vision is not totally physiological and can be influenced by training, if the eyes are not defective. Repetitive procedures can be utilized to develop strong convergent vision. Athletes, such as baseball shortstops, have well-developed convergent vision. In every movement, the two eyes have to translate in unison to preserve binocular vision, with an accurate and responsive neuromuscular apparatus that is not usually subject to fatigue, controlling their motility and coordination. Changes in ocular convergence or head motion are considered in the calculations made by the complex ocular system to produce the proper neural inputs to the eye muscles. An eye movement of 10 degrees may be completed in about 40 milliseconds, with the calculations occurring faster than the eye can reach its intended target. Small eye movements are known as saccades and the larger movements from one point to another are termed versions.

The human visual system must not only detect light and color, but as an optical system, must be able to discern differences among objects, or an object and its background. Known as physiological contrast or contrast discrimination, the relationship between the apparent brightness of two objects that are seen either at the same time (simultaneous contrast) or sequentially (successive contrast) against a background, may or may not be the same. In the human visual system, contrast is reduced in environmental darkness and with individuals suffering from color visual deficiencies such as red-green color blindness. Contrast is dependent on binocular vision, visual acuity, and image processing by the visual cortex of the brain. An object with low contrast, which cannot be distinguished from the background unless it is moving, is considered camouflaged. However, colorblind individuals are often able to detect camouflaged objects because of increased rod vision and loss of misleading color cues. Increasing contrast translates into increased visibility, and a quantitative numerical value for contrast is usually expressed as a percentage or ratio. Under optimal conditions, the human eye can barely detect the presence of two percent contrast.

With human vision, an apparent increase in contrast is perceived in a narrow zone on each side of the boundary between two areas of different brightness and/or chromaticity. At the end of the nineteenth century, French physicist Michel Eugéne Chevreul discovered simultaneous contrast. As a special function of human visual perception, the edges or contour of an object are highlighted, setting the object away from its background and easing spatial orientation. When positioned over a bright background, the region at the edge of a dark object appears lighter than the rest of the background (in effect, the contrast is enhanced). With this perception phenomenon, the color with the strongest contrast, the complementary color, is created (by the brain) at the edge. Because the color and its complement are perceived simultaneously, the effect is known as simultaneous contrast. Borders and other lines of demarcation that separate the contrasting areas tend to lessen the effect (or optical illusion) by eliminating marginal contrast. Many forms of optical microscopy, most notably phase contrast illumination, take advantage of these features of the human visual system. By increasing the physical contrast of an image without having to change the object via staining or other technique, the phase contrast specimen is protected from damage or death (in the case of living specimens).

The spatial frequency response of the human eye can be evaluated by determining the ability to detect a series of strips in a modulated sinusoidal grating. Test gratings feature alternating regions (strips) of light and dark, which increases linearly from higher to lower frequencies along the horizontal axis while contrast decreases logarithmically from top to bottom. The boundary of stripes that can just be distinguished by individuals with normal vision is between 7 and 10 cycles per degree. For achromatic vision, when the spatial frequency is very low (broad line spacing), high contrast is required to detect the sinusoidally varying intensity. As the spatial frequency rises, humans can detect periods with less contrast, reaching a peak of about 8 cycles per degree in the visual field. Beyond that point, higher contrast is again required to detect the finer sinusoidal stripes.

Examination of the modulation transfer function (MTF) of the human visual system reveals that the contrast necessary to detect the luminance variation in standardized sinusoidal gratings increases at both higher and lower spatial frequencies. In this regard, the eye behaves quite differently from a simple imaging device (such as a film camera or CCD sensor). The modulation transfer function of a simple, focused camera system displays a maximum modulation at zero spatial frequency, with the degree of modulation dropping more or less monotonically to zero at the camera's cutoff frequency.

When the luminance of a scene fluctuates periodically several times a second (as it does with television and computer monitor screens), humans perceive an irritating sensation, as though the sequential scenes were disjoined. When the fluctuation frequency increases, irritation increases and reaches a maximum at around 10 hertz, especially when bright flashes of illumination alternate with darkness. At higher frequencies, the scene no longer appears disjointed, and objects displaced from one scene to the next are now perceived to be moving smoothly. Commonly referred to as flicker, the annoying light fluttering sensation can persist up to 50-60 hertz. Beyond a certain frequency and luminance, known as the critical flicker frequency (CFF), screen flicker is no longer perceived. This is the primary reason why increasing the refresh rate of a computer monitor from 60 to 85-100 hertz produces a stable, flicker-free display.

Advances in semiconductor fabrication technology, especially complementary metal oxide semiconductors (CMOS) and bipolar CMOS (BiCMOS) techniques, has led to a new generation of miniature photosensors that feature extraordinary dynamic range and fast response. Recently, arrays of CMOS sensor chips have been arranged to model the operation of the human retina. These so-called eye chips, by combining optics, human vision, and microprocessors, are advancing ophthalmology through the new field of optobionics. Damaged retinas resulting from debilitating visual diseases, such as retinitis pigmentosa and macular degeneration, as well as aging and injuries to the retina, which rob vision, are being corrected with the implanted eye chips. The silicon eye chips contain approximately 3,500 miniature light detectors attached to metal electrodes that mimic the function of the human rods and cones. The light detectors absorb incident light refracted by the cornea and lens and produce a small quantity of electrical charge that stimulates the retinal neurons. Featuring a diameter of two millimeters (see Figure 9), the replacement retina is half as thick as a typical piece of paper and is implanted into a pocket under the damaged retina.

As an alternative to the eye chip, a retinal prosthesis using a digital signal processor and a camera mounted on a pair of glasses, captures and transmits an image of an object or scene. Wirelessly, the image is sent to an embedded receiver chip near the retinal layers where nerve impulses are sent to the brain. Artificial retinas, however, will not treat glaucoma or vision deficiencies that damage the nerve fibers leading to the optic nerve. As optobionics advances, so does science's understanding of the complex human visual system.

Contributing Authors

Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.

Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.


How can a clear object be transparent and visible at the same time?

Clear objects are visible because they bend the light as it passes through. There are four basic things that can happen to light when it hits an object:

  1. Specular Reflection: Think of a mirror or metal spoon. The light bounces off the object's surface like a billiard ball, allowing the original image to be seen in the object.
  2. Diffuse Reflection: Think of raw wood, flowers, or non-glossy painted surfaces. The light bounces off the object's surface in all directions, revealing the shape and color of the object.
  3. Absorption: Think of a black piece of coal or ash. The light enters the object where it is absorbed and converted to heat.
  4. Transmission/Refraction: Think of a glass of water. The light travels straight through the object but the direction it is traveling bends when entering and leaving the object.

In reality, all materials interact with light in all four ways. For instance, consider the hood of a red sports car. Some of the light is reflected specularly (leading to the glare spots you see and the image of trees reflected off the car). Some of the light is reflected diffusely (leading to the red color you see). Some of the light is absorbed (leading to the orange, yellow, green, blue, and violet light you don't see because it is absorbed – if these colors were not absorbed, the car would look white and not red). Also, some of the light is transmitted/refracted (very little actually).

For many materials, there may be one dominant way it interacts with light, so that the other ways are so small that they can be ignored. For instance, water does indeed absorb some red light (that is why the ocean is blue), and water does indeed reflect some light (that is why there is glare from the sun on the water's surface), but for the most part we can think of water as a clear material because transmission/refraction dominates.

Now, the interesting part is that each of the four interactions listed above alters the light. Our brains are able to detect this alteration in the light and deduce the presence and shape of an object from this information. Strictly speaking, we never see an "object". We see "light" that has been altered by an object. That is why it is so difficult to build machines that can see the way humans do: there is a great deal of intelligence required to deduce an object's shape and location from a pattern of light that it has altered.

When it comes to clear objects, we see them because we see the way light bends (refracts) as it passes through the objects. Look closely at a glass cup. When you look at the glass cup, what do you see? You just see an image of whatever is behind the cup, but distorted. Refraction bends the light as it passes through the cup and the background image ends up changed. Your brain is smart enough to be able to deduce the shape of the cup simply by how the background image is distorted.

This leads us to an interesting notion. If the refraction of a a clear material can be mostly canceled, the object can be made virtually invisible. One way to cancel refraction effects is to shape a clear material into a very flat slab with parallel surfaces. When light enters the slab, it bends, but when it leaves the slab out the other side it bends back by the same amount. As a result, the image coming out the other side is undistorted and the slab is effectively invisible. This is the principle behind windows. Windows are made out of clear glass and fashioned to be very flat, so that you can't actually see the window. You see the landscape beyond the window as if the window were not there (windows are not completely invisible because they do reflect a small amount of light which can be detected under the right conditions).


Night Vision And Humans: Why Can't We See Color?

When we are in a fairly dark room, or outside at night away from lights, we can still see, but we can't see the colors of things very well. Why is that?

Sensing Light

There are two kinds of light-sensitive organs located in the backs of our eyes: rod-shaped and cone-shaped. Both rods and cones are sensitive to light. The difference between them is that the rods allow us to see in very dim light but don't permit detection of color, while the cones let us see color but they don't work in dim light.

When it gets dark the cones lose their ability to respond to light. The rods continue to respond to available light, but since they cannot see color, so to speak, everything appears to be various shades of black and white and gray.

Dim Light

A curious thing is that in dim light you can see more clearly out of the side of your eye, because the light-sensitive rods are more highly concentrated off to the side in the back of your eye.

So, next time you're out on a clear night, notice how little color you can see, and how you can see objects like dim stars better out of the corner of your eye than from the center.


Mixing colours

The primary colours of light are red, green and blue. Mixing these colours in different proportions can make all the colours of the light we see. This is how TV and computer screens work. If you look at a screen with a magnifying glass you will be able to see that only these three colours are being used. For example, red and green lights are used to make our brain perceive the image as yellow.

When coloured lights are mixed together, it is called additive mixing. Red, green and blue are the primary colours for additive mixing. If all of these colours of light are shone onto a screen at the same time, you will see white.

This is different when you are mixing paints. Each colour of paint is absorbing certain colours and reflecting others. Each time another colour of paint is mixed in, there are more colours absorbed and less are reflected. The primary colours for adding paints or dyes, such as for a computer printer, are yellow, magenta and cyan. If you mix all of these colours together, you will absorb all the light and will only see black, because no light will be reflected back to your eyes.

You can easily experiment with this. Hold some coloured cellophane in front of your eyes and have a look around. Notice how some colours are changed and others look similar. Figure out which colours are being absorbed.


Light Absorption and Color Filters

When white light shines on a red object, all of the colors that form the white light are absorbed except red, which is reflected. This is why the object appears red. A filter is a transparent material that absorbs some colors and allows others to pass through.

Light is the only source of color. Color pigments (paints, dyes, or inks) show color by absorbing certain parts of the light spectrum and reflecting the parts that remain. Color filters work the same way, absorbing certain wavelengths of color and transmitting the other wavelengths.

A yellow color filter will let through only yellow and absorb all other colors. So when blue light is allowed through a blue filter onto a blue object, the object will still reflect blue and therefore appear blue. But when blue light from a blue filter hits a red object, the blue will be absorbed and no light will be reflected, giving the object an appearance of being black.

Materials

  • Flashlight
  • Red, blue, and green construction paper
  • See-through colored cellophane paper
  • Camera filters in red, blue and green
  • Masking tape or a rubber band

Research Questions

  • Why did the papers look white, red, blue, and green (respectively) in white light?
  • How did the filters affect the white flashlight beam?
  • Why did the yellow and green papers seem to lose their color when red light was shined on them?

Procedure

  1. Darken the room as much as possible.
  2. Turn on the flashlight and aim it at the white paper. Observe and record the color of the paper in the data table.
  3. Repeat step 2 with the red, blue, and green pieces of paper.
  4. Place the red filter in front of the beam of the flash light as shown using tape or a rubber band to secure the cellophane paper filter. Shine the filtered beam on the white, red, blue, and green papers and record the colors seen.
  5. Repeat using the blue filter and then the green filter. After each test, record the results.

Digging Deeper

Place a filter in front of the light source. Combine two colored filters. Now combine three colors. Experiment with many different combinations.

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Definitions of Black and White

The correspondence of a color to a specific wavelength is called spectral color. White and black are excluded from this definition because they do not have specific wavelengths. White is not defined as a color because it is the sum of all possible colors. Black is not defined as a color because it is the absence of light, and therefore color. In the visual art world, white and black may sometimes be defined as distinct colors. This is different from the concept of spectral color in physics.


"Blue" Cone Distinctions

The "blue" cones are identified by the peak of their light response curve at about 445 nm. They are unique among the cones in that they constitute only about 2% of the total number and are found outside the fovea centralis where the green and red cones are concentrated. Although they are much more light sensitive than the green and red cones, it is not enough to overcome their disadvantage in numbers. However, the blue sensitivity of our final visual perception is comparable to that of red and green, suggesting that there is a somewhat selective "blue amplifier" somewhere in the visual processing in the brain.

The visual perception of intensely blue objects is less distinct than the perception of objects of red and green. This reduced acuity is attributed to two effects. First, the blue cones are outside the fovea, where the close-packed cones give the greatest resolution. All of our most distinct vision comes from focusing the light on the fovea. Second, the refractive index for blue light is enough different from red and green that when they are in focus, the blue is slightly out of focus (chromatic aberration). For an "off the wall" example of this defocusing effect on blue light, try viewing a hologram with a mercury vapor lamp. You will get three images with the dominant green, orange and blue lines of mercury, but the blue image looks less focused than the other two.


How Vision Works

It's no accident that the main function of the sun at the center of our solar system is to provide light. Light is what drives life. It's hard to imagine our world and life without it.

The sensing of light by living things is almost universal. Plants use light through photosynthesis to grow. Animals use light to hunt their prey or to sense and escape from predators.

­Some say that it is the development of stereoscopic vision, along with the development of the large human brain and the freeing of hands from locomotion, that have allowed humans to evolve to such a high level.In this article, we'll discuss the amazing inner workings of the human eye!

Although small in size, the eye is a very complex organ. The eye is approximately 1 inch (2.54 cm) wide, 1 inch deep and 0.9 inches (2.3 cm) tall.

The tough, outermost layer of the eye is called the sclera. It maintains the shape of the eye. The front sixth of this layer is clear and is called the cornea. All light must first pass through the cornea when it enters the eye. Attached to the sclera are the muscles that move the eye, called the extraocular muscles.

The choroid (or uveal tract) is the second layer of the eye. It contains the blood vessels that supply blood to structures of the eye. The front part of the choroid contains two structures:

  • The ciliary body - The ciliary body is a muscular area that is attached to the lens. It contracts and relaxes to control the size of the lens for focusing.
  • The iris - The iris is the colored part of the eye. The color of the iris is determined by the color of the connective tissue and pigment cells. Less pigment makes the eyes blue more pigment makes the eyes brown. The iris is an adjustable diaphragm around an opening called the pupil.

The iris has two muscles: The dilator muscle makes the iris smaller and therefore the pupil larger, allowing more light into the eye the sphincter muscle makes the iris larger and the pupil smaller, allowing less light into the eye. Pupil size can change from 2 millimeters to 8 millimeters. This means that by changing the size of the pupil, the eye can change the amount of light that enters it by 30 times.

The innermost layer is the retina -- the light-sensing portion of the eye. It contains rod cells, which are responsible for vision in low light, and cone cells, which are responsible for color vision and detail. In the back of the eye, in the center of the retina, is the macula. In the center of the macula is an area called the fovea centralis. This area contains only cones and is responsible for seeing fine detail clearly.

The retina contains a chemical called rhodopsin, or "visual purple." This is the chemical that converts light into electrical impulses that the brain interprets as vision. The retinal nerve fibers collect at the back of the eye and form the optic nerve, which conducts the electrical impulses to the brain. The spot where the optic nerve and blood vessels exit the retina is called the optic disk. This area is a blind spot on the retina because there are no rods or cones at that location. However, you are not aware of this blind spot because each eye covers for the blind spot of the other eye.

When a doctor looks at the back of your eye through an ophthalmoscope, here's the view:

Inside the eyeball there are two fluid-filled sections separated by the lens. The larger, back section contains a clear, gel-like material called vitreous humor. The smaller, front section contains a clear, watery material called aqueous humor. The aqueous humor is divided into two sections called the anterior chamber (in front of the iris) and the posterior chamber (behind the iris). The aqueous humor is produced in the ciliary body and is drained through the canal of Schlemm. When this drainage is blocked, a disease called glaucoma can result.

The lens is a clear, bi-convex structure about 10 mm (0.4 inches) in diameter. The lens changes shape because it is attached to muscles in the ciliary body. The lens is used to fine-tune vision.

Covering the inside surface of the eyelids and sclera is a mucous membrane called the conjunctiva, which helps to keep the eye moist. An infection of this area is called conjunctivitis (also called pink eye).

The eye is unique in that it is able to move in many directions to maximize the field of vision, yet is protected from injury by a bony cavity called the orbital cavity. The eye is embedded in fat, which provides some cushioning. The eyelids protect the eye by blinking. This also keeps the surface of the eye moist by spreading tears over the eyes. Eyelashes and eyebrows protect the eye from particles that may injure it.

Tears are produced in the lacrimal glands, which are located above the outer segment of each eye. The tears eventually drain into the inner corner of the eye, into the lacrimal sac, then through the nasal duct and into the nose. That is why your nose runs when you cry.

There are six muscles attached to the sclera that control the movements of the eye. They are shown here:


Colored Shadows

When lights of different colors shine on the same spot on a white surface, the light reflecting from that spot to your eyes is called an additive mixture because it is the sum of all the light. We can learn about human color perception by using colored lights to make additive color mixtures.

Video Demonstration

Tools and Materials

  • Red, green, and blue lightbulbs
  • A way to plug in all three lightbulbs at the same time and simultaneously direct their light onto the same white surface
  • A white surface, such as a wall or a piece of white poster board (white paper taped to stiff cardboard also works well)
  • Any narrow solid object such as a pencil or ruler (not pictured)

Assembly

  1. Set up the bulbs and the white surface, which will be your screen, in such a way that the light from all three bulbs falls on the same area of the screen and all bulbs are approximately the same distance from the screen.
  2. For best results, put the green bulb between the red and blue bulbs.

To Do and Notice

Make the room as dark as possible. Then turn on the three colored lights, aim them all at your white screen, and adjust the positions of the bulbs until you obtain the “whitest” light you can make on the screen.

Place a narrow opaque object, such as a pencil, fairly close to the screen. Adjust the distance until you see three distinct colored shadows on the screen.

Remove the object, turn off one of the colored lights, and notice how the color on the screen changes. Put the object in front of the screen again and notice the colors of the shadows. Move the object close to the screen until the shadows overlap. Notice the color of the combined shadows.

Repeat the preceding step with a different bulb turned off while the other two remain on, and then a third time until you’ve tried all the possible combinations. Repeat again with only one color turned on at a time, and then with all three on. Vary the size of the object and the distance from the screen. Try using your hand as an object.

What’s Going On?

Your retina, which covers the back of the eye, contains light receptors called rods and cones. Rods are used for night vision and they only let you see in shades of gray. You have only one type of rod but three types of cones. Cones let you see in color as long as it's not very dark.

All three types of cones respond to a wide range of wavelengths, but one type is the most sensitive to long wavelengths (the red end of the spectrum), one to medium wavelengths, and one to short wavelengths (the blue end of the spectrum). With just these three types of cones, we are able to perceive more than a million different colors.

When a red light, a blue light, and a green light are all shining on the screen, the screen looks white because these three colored lights stimulate all three types of cones in your eyes approximately equally, creating the sensation of white. Red, green, and blue are therefore called additive primaries of light.

With these three lights you can make shadows of seven different colors—blue, red, green, black, cyan, magenta, and yellow—by blocking different combinations of lights (click to enlarge diagram below). When you block two lights, you see a shadow of the third color—for example, block the red and green lights and you get a blue shadow. If you block only one of the lights, you get a shadow whose color is a mixture of the other two. Block the red light and the blue and green light mix to create cyan block the green light and the red and blue light make magenta block the blue light and red and green make yellow. If you block all three lights, you get a black shadow.

You can achieve a similar effect by turning off different lightbulbs. If you turn off the red light, leaving on only the blue and green lights, the entire screen will appear cyan. And when you hold an object in front of the screen, you will see two shadows, one blue and one green. In one place, the object blocks the light coming from the green bulb, leaving a blue shadow in the other location it blocks the light from the blue bulb, leaving a green shadow.

When you move the object close to the screen, the shadows overlap, leaving a very dark (black) shadow where the object blocks both lights. When you turn off the green light, leaving on the red and blue lights, the screen will appear to be magenta, a mixture of red and blue. The shadows will be red and blue. When you turn off the blue light, leaving on the red and green lights, the screen will appear to be yellow. The shadows will be red and green.

It may seem strange that a red light and a green light mix to make yellow light on a white screen. It just so happens that a particular mixture of red and green light stimulates the cones in your eyes exactly as much as they’re stimulated by yellow light—that is, by light from the yellow portion of the rainbow—so your eye can't tell the difference. Whether a mixture of red and green light or yellow light alone—whenever the cones in your eye are stimulated in just these proportions, you'll see the color yellow.

Going Further

If you let light from the three bulbs shine through a hole in a card that is held an appropriate distance from the screen, you will see three separate patches of colored light on the screen, one from each lamp. (Make the hole large enough to get a patch of color you can really see.) If you move the card closer to the screen, the patches of light will eventually overlap and you will see the mixtures of each pair of colors.

If you want to experiment further, find out what happens when you use different colors of paper or poster board for the screen. Try yellow, green, blue, red, or purple paper, and so on.

Resources

Watch this video to see Teacher Institute staff present this activity in a workshop designed to help teachers bring Science Snacks into the classroom.