### Re: Why is light White?

Area: Physics
Posted By: Gaby Hayon, Phd student at the Neural computation department
Date: Thu May 1 06:21:48 1997
Area of science: Physics
ID: 861062081.Ph
Message:

Hi John,
Before answering your question, lets try to understand how do we see things, and then talk about colors. We see an object because light is scattered by that objects and reaches our eye. We will see objects as black if no light is reflected from them (We accually don't see them, we just see their surround). That's most of the idea behind black and white vision, now let's discuss colors.
As you probably know, all kind of light (and other kinds of radiation like radio waves, and microwaves) are electro-magnetic waves which travel at the velocity of light. Different colors of light means waves of different wave length (purple has the shortest wave length, and red the longest). An object will look red if it absorbs light of all wavelengths other then the red light. So that's colored light, but how do we see it ? To understand how people see colors, we'll go to the eye. At the eye there are four kinds of receptors:
1. The first one is sensitive to all the light of all visible wavelengths (red till purple), this receptor gives us the black and white vision.
2. The other three receptors are sensitive to a narrow strip of light wavelengths. Those strips are centered around the red-green-blue colors.

Lets try to make this clear by using a few examples:
1. Seeing a red ball: Light is reflected from the ball. The ball absorbs all colors other then red. Light of red wavelength reaches the eye, turns on the appropriate photoreceptors, and the brain receives a message that the ball color is red. If we use a blue light to light the ball it will be seen as black (the eye will recive no light from the ball).
2. Seeing a white board: If from very close points (or the same point) in space one receives red, green and blue light, photoreceptors for the three colors in close points on the retina are activated. Then the color perception system checks the relative magnitude of the three "colors", and the person perceives a color. In the white color case all three colors should be of the same magnitude.

To sum things up:
1. One sees black when no light is observed by the eye.
2. There are receptors for RED-GREEN-BLUE colors.
3. All colors are electro-magnetic light waves (of different wavelengths).
4. WHITE is a linear combination of RED-GREEN-BLUE, with equal weights.
5. Any other color is a linear combination of the above three colors with different weights.
You may find additional help at the following references:
1. A short page about color perception
2. An explanation about how how we combine the three primitive colors, may be found in Interactive Illustrations of Color Perception
3. How Do Animals See ?
4. A recommanded web book about visual perception.
Yours, Gaby.

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