Re: why do we see white when blue, red, and green light rays are conbined?

Light comes in different wavelengths, which we see as different colors. The range of wavelengths that the human eye can see extends from around 400 nanometers, which we perceive as violet, to around 700 nanometers, which looks red to us.

Although there are an infinity of different wavelengths in this range, the human system for color vision detects them with only three different kinds of light-detecting molecules. When one of these molecules captures a photon of light, a signal is generated in a nearby nerve. When a large number of such nerve signals reach your brain, you get a sensation of color. One kind of light-detecting molecule is particularly good at capturing photons with short wavelengths (the blue end of the spectrum), while the other two are good at capturing photons of red and green light.

Thus, the brain's perception of color is based entirely on the amounts of light captured by the blue-sensitive, red-sensitive, and green-sensitive pigment molecules in your eye. Two very different mixtures of wavelengths appear as exactly the same color if they cause the same amounts of light to be captured by the blue-sensitive pigment molecules, the same amounts to be captured by the green-sensitive pigment molecules, and the same amounts to be captured by the red-sensitive molecules. Color TVs make use of this fact when they mix just three colors, in different amounts, to make all the colors you see on TV. (With a magnifying glass, you can see the individual red, green, and blue color dots on the screen of a color TV.)

Getting back to the original question, a mixture of red, green, and blue light will look just like white light (an equal mixture of all wavelengths) when the three kinds of light-detecting molecules in your eye are catching the same amounts of light as if the light were white.

A couple of asides:

When it is dark, your eyes use completely different light-sensitive cells and a completely different kind of light-sensitive molecule. Your night vision uses only one kind of light-sensitive molecule, and this is why it's so hard to distinguish colors in dim light.

Color blindness results from a mistake in your genes' recipe for one or more of the three light-detecting molecules. For example, if the cells in your eye can't make the red-sensitive pigment molecule, you will be unable to distinguish between colors in the red-orange-yellow-green part of the spectrum.

Some people actually have genes for four different light-detecting molecules. I have never seen an account of the color-distinguishing abilities of such people.

References:

An advanced, technical web page on color vision.

An excellent description of color vision appears in Volume I of The Feynman Lectures on Physics, by Richard P. Feynman, Robert B. Leighton, and Matthew Sands, Addison-Wesley (1963).