Additive Properties of Light

 

To see colors, there must be at least a little bit of light. White light is in fact made of all colors, but the colors we see depend on the quantity and color of light being thrown back (reflected) or taken in (absorbed) by an object. In order for humans to see color, the reflected light goes to the sensitive area in the back of our eye called the retina which sends impulses to the brain which then interprets the combination of impulses to be the color human see.

The retina has two kinds of cells that respond to color: rods and cones. The rods are sensitive to light intensity or brightness, and they don't respond to color. It is the rods that allow us to see in low light situations. For example, when light strikes our blue jeans, the blue jeans absorb all colors of light, other than blue. The blue we see is the blue light reflected off of the jeans. But we can not be able to tell that our jeans are blue in the dark!

The RETINA, which is the inner lining of the eye, is the receiving or light-sensitive portion. The retina contains delicate light-sensitive nerve fiber endings called cones and rods. The RODS are more sensitive to light than the cones; they function primarily during "night" vision. Whereas cones are linked one-to-one with nerve endings, a bundle of rods (many thousand) is served by one nerve ending. These multiple hookups result in very poor visual acuity. However, since each nerve ending serves a relatively large portion of the visual field, the rod system is more likely to detect changes such as motion or flicker. This accounts for the phenomenon of seeing flicker out of the "corner of the eye" which disappears when one turns to face the source, since the periphery of the visual field is where rods are the predominant photoreceptor. Similarly, in dim light situations one is less likely to detect an object by looking straight at it than by placing the object in the periphery where the more sensitive cones can detect it.
There is no color response with the rod system. Rods produce a black and white response, which is actually a reaction to variations in luminance. Moving from a very light environment into a dark environment results in a change in sensitivity of the visual system due to dark adaptation. Dark adaptation is not completely achieved until about 1 hour after light is removed. The initial rapid phase of dark adaptation is due to low-level cone action, while the remaining portion of dark adaptation is due to slower rod action. Scotopic or rod vision occurs when the eye has been dark adapted and only the rods are functioning.
The CONES are the photoreceptors that are effective for "daylight" vision only. The maximum concentration of cones is in the fovea. Cone density decreases with increasing distance from the fovea. The fovea is inactive under very dim light (less than 0.003 candela per square foot (cd/ft2)). The candela per square foot is a unit of luminous energy leaving a surface and arriving at the retina. The rods must take over the visual process during low levels of illumination. When the eye is receiving light at levels above approximately .3 cd/ft2, the system is said to be operating under photopic or pure cone vision.
While moving from a dark environment into a very light environment, the visual system experiences a change in sensitivity. this phenomena is called light adaptation. Light adaptation involves primarily the cone system, and usually takes less than a minute.

Color and Rainbows:
In order to see a color, the color must be there for us to see, so if there is red in a rainbow, then the red must be there for us to see. Sunlight, or white light, is a combination of all of the colors of the rainbow. The colors of a rainbow are red, orange, yellow, green, blue, indigo, and violet. The order of the colors is from longest to shortest wavelength and each of these colors is present in white light, but our eye can't notice the individual colors. Human eye sees only the mixture which interprets as white by the brain. A rainbow forms as sunlight strikes the raindrops and passes through each raindrop and diffracts, or bends, slightly. Each color (wavelength) in the sunlight diffracts a different amount. Red diffracts the least while violet diffracts the most. Once the light hits the back of the water drop, it is reflected back out through the front of the drop. Human eye can then see the colors in the rainbow as the refracted and reflected light from millions of water drops enters the eye.
This division of white light into a rainbow of colors is called a spectrum. In every rainbow, red is on the top of the arch, followed by orange, yellow, green, blue, and indigo. Violet is on the bottom (picture below).

Each drop of water in the rainbow reflects the entire spectrum of color, but human eye is not in a position to see the entire spectrum from each drop. The only part human sees is the part of the spectrum which strikes the retina of our eye. However, we see all the colors in a rainbow because each color is coming from different water droplets at different elevations.
A rainbow example is a proof that white light, or sunlight, is made of colors.