How the human sees :


White is a mixture of many () and cells react differently to different wavelengths.

Light is part of a range of called As you move through the of the . visible light from to , the • Color is how we perceive the of light. energy of the light increases. • All of the colors of visible light have different . • Red light has the lowest energy and violet light has the highest energy.

Our have two types of Cone cells respond to color and there are photoreceptors: cone cells and rod cells. three types.

• One type responds best to red light. • Another type responds best to light and the last type responds best to light.

1 We see a wide range of colors Rod cells respond only to differences in depending on how each kind of cone light , and not to color. is stimulated. • Rod cells detect , , and shades of • For example, we see white light when all gray. three types of cones (red, green, blue) are • However, rod cells are more sensitive than equally stimulated. cone cells especially at low light levels.

At night, colors seem washed out An average contains about because there is not enough light for 130 million rod cells and 7 million cone cone cells to . cells.

• When the light level is very dim, you see • Each one contributes a “dot” to the total “” images transmitted from imaggyye assembled by your . your rod cells. • The brain evaluates all 137 million “dots” about 15 each second.

The cone cells are concentrated near the Our eyes work according to an additive center of the , making color process. best at the center of the eye’s field of • Three photoreceptors (red, green, and blue) view. in the eye operate together so that we see millions of different colors. • EhEach ll“l “colors” ”hi the lfs from th e surrounding rod cells.

2 The color you “see” depends on how We perceive different colors as a much energy is received by each of the combination of percentages of the three three different types of cone cells. additive primary colors: red, green, and blue. • The brain thinks “green” when there is a • For example, we see when the brain strong signal from the ggqyggets an equally strong signal from both the green cone cells but no red and the green cone cells at the same signal from the blue or . red cone cells. • Whether the light is actually yellow, or a combination of red and green, the cones respond the same way and we perceive yellow.

If the red signal is stronger than the green signal we see .

If all three cones send an equal signal to the brain,,pg we interpret the light we see as white.

The human eye can see any color by You can also see pure yellow light or adding different percentages of the three orange light that is not a mixture of red additive primary colors. and green.

• Mixing red and green light is one way the • For example, eye sees the color yellow or orange, for street liggphts produce example. pure yellow light, not • Keep in that you perceive these colors a mixture of yellow even though the light itself is still red and and green. green.

3 Not everyone sees color the same way. Although can be caused by , it is often an • A condition called color blindness affects inherited condition. about 8 percent of males and 0.4 percent of females. • More males than females have color • This means that about one out of every 13 blindness because men has color blindness and about one out of how the genes of every 250 women has color blindness. that determine our sex are inherited.

Males have a X and a Y ; People who are color blind have trouble females have two X . seeing certain colors.

• The color blindness alleles are on the X • The most common chromosome which males receive only condition is red-green from their mothers; they receive the Y color blindness. chromosome from their fathers. • People with this type of • Because females receive two X color blindness have chromosomes, they have two chances to trouble seeing and inherit the alleles for normal color vision. .

Less common is blue-green color For example, color is extremely blindness. important when driving because traffic and street signs are color-coded. • Complete color blindness means that the person can only see . • Fortunately, in most • Fortunately, this condition is rare. states, the traffic lights • It is easy to lead a normal life with color are vertical and the colors blindness. are in the same position— red on top, yellow in the • Having color blindness just means that an center, and green on the individual must look for ways to adapt to bottom. situations where color is involved.