Color can be thought of as a psychological and physiological response to light waves of a specific frequency or set of frequencies impinging upon the eye.
Our perception of color arises from the composition of light - the energy spectrum of photons - which enter the eye. Our eyes are sensitive to a very narrow band of frequencies within the enormous range of frequencies of the electromagnetic spectrum.
This narrow band of frequencies is referred to as the visible light spectrum. Visible light - that which is detectable by the human eye - consists of wavelengths ranging from approximately 780 nanometer (7.80 x 10-7 m) down to 390 nanometer (3.90 x 10-7 m). Specific wavelengths within the spectrum correspond to a specific color based upon how humans typically perceive light of that wavelength. The long wavelength end of the spectrum corresponds to light which is perceived by humans to be red and the short wavelength end of the spectrum corresponds to light which is perceived to be violet. Other colors within the spectrum include orange, yellow, green, blue and indigo.
The Eye and Color Sensation
An understanding of the human response to color demands that one understand the biology of the eye. The retina on the inner surface of the back of the eye contains photosensitive cells. These cells contain pigments which absorb visible light. Of the two classes of photosensitive cells, rods and cones, only the cones allow us to distinguish between different colors. The rods are effective in dim light and sense differences in light intensity. In dim light we perceive colored objects as shades of grey, not shades of color.
A cross-sectional representation of the eye showing light entering through the pupil. The photosensitive cells, cones and rods, are located in the retina: cones respond to color and rods respond to light intensity.
Cones Red, Blue and Green of the Retina
Color is perceived in the retina by three sets of cones which are photoreceptors with sensitivity to photons whose energy broadly overlaps the blue, green and red portions of the spectrum. Color vision is possible because the sets of cones differ from each other in their sensitivity to photon energy. The maximum sensitivity is to yellow light, but cone R has a maximum in the red-orange, G in the green-yellow, and B in the blue. The sensitivities of the three comes overlap. For every color signal or flux of photons reaching the eye, some ratio of response within the three types of cones is triggered. It is this ratio that permits the perception of a particular color.
The response of the tree cones to incident light: cone R (pigment R) has a maximum sensitivity in the orange-red, cone G (pigment G) in the green-yellow, and cone B (pigment B) in the blue portions of the visible spectrum. The sensitivities of the three cones overlap and the perceived color is due to the relative response of the three cones.
In the same manner, the green cone is most sensitive to wavelengths of light associated with the color green; yet the green cone can also be activated by wavelengths of light associated with the colors yellow and blue. The graphic is a sensitivity cone which depicts the range of wavelengths and the sensitivity level for the three kinds of cones. The cone sensitivity curve shown above helps us to better understand our response to the light which is incident upon the retina. While the response is activated by the physics of light waves, the response itself is both physiological and psychological.
The human visual system is logarithmic and not linear. For example, a beam of light that contains mostly short-wavelength blue radiation stimulates the cone cells that respond to 430-nanometer light far more than the other two cone types, activating the blue color pigment, 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 seen as red. 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 only able to distinguish between bluish and greenish color components. In contrast, some primates, most notably humans, exhibit trichromatic color vision, splitting the red, green and blue light stimuli.
Similar Representaion of Cone Response where; S = Short, M = Medium amd L = Long in Reference to Wavelenght.
Suppose that white light - i.e., light consisting of the full range of wavelengths within the visible light spectrum - is incident upon the retina. Upon striking the retina, the physiological occurs: photochemical reactions occur within the cones to produce electrical impulses which are sent along nerves to the brain." Upon reaching the brain, the psychological occurs: the brain detects the electrical messages being sent by the cones and interprets the meaning of the messages. The brain responds by saying "it is white." For the case of white light entering the eye and striking the retina, each of the three kinds of cones would be activated into sending the electrical messages along to the brain. And the brain recognizes that the messages are being sent by all three cones and somehow interprets this to mean that white light has entered the eye.
The Visible Spectrum
Figure 5. White light composed of all wavelengths of visible light incident on a pure blue object. Only blue light is reflected from the surface.