Perceived color
Even with the rainbow explained, the puzzle of color still baffled scientists. For instance, children experimenting with crayons usually find that a combination of yellow and blue produces green. Is green then a basic color as the rainbow suggests, or is it the combination of two other such colors?
The riddle was solved around 1860 by James Clerk Maxwell (pictured on the left), the brilliant Scottish physicist who also gave us the basic equations of electricity, the ones that predicted electromagnetic waves (see Section S-5). Maxwell showed, while still a student, that two kinds of color existed, depending on whether it was perceived by an instrument or by the human eye:
- "Spectral color, " i.e. the colors of the rainbow and their combinations. The amount which each part of the rainbow spectrum contributes to a beam of light can be determined by splitting the beam with a prism.
- "Perceived color" reported by the human eye to the brain.
An instrument using prisms ("spectrograph") will reveal that the green color in the rainbow and the green formed by (yellow + blue) are not the same. However, the human eye cannot tell the difference.
Our eye contains three kinds of light-sensitive cells, each sensing a different band of colors--one band centered in the red, one in the green (and yellow) and one in the blue. When we see green, the blue-sensitive and green-yellow sensitive cells are both stimulated; but our eyes cannot tell whether that happens because we see both all colors mixed together, or because we see just one color (rainbow green) that is halfway between those two color bands.
Any color which we see--including brown, olive-green and others absent in the rainbow--is an impression our brain conveys as it combines signals from these 3 color bands. Color-blind persons do not have some types of eye cells, and their world lacks certain colors, or even (for those with only one kind of cell) any color at all.
That is why color TV and color printers can be based on the three "primary colors" red, green and blue. These devices do not in any way reproduce the true spectral color of the objects they show, but they are still capable of representing any color our eyes can see. Here and at the bottom of this page you can link to a program letting you experiment with 3-color combinations, using the color monitor of your computer.
The Spectrum
Any color discussed from now on will be a spectral color. Two kinds of color distributions are important in nature:
(1) In light emitted from solids, liquids or extensive bodies of dense gas such as the Sun, the colors are distributed continuously. Their exact distribution ("black body spectrum") depends on the temperature at which it is produced--a warm hand radiates mostly in the infra-red, a glowing bar of iron is cherry-red, a lightbulb filament is bright yellow, and sunlight is white-hot.
[Also of this type is the distribution of microwave radiation left over from the "big bang" when the universe apparently began, a radiation observed by NASA's COBE satellite, the Cosmic Background Explorer. When the observed COBE spectrum was first shown before a meeting of astronomers, it caused a great stir. Observed values generally show some experimental error, but here they were so close to the predicted theoretical curve that the first impression of the viewers was that the presenters had drawn the curve first and then placed their points on top of it.]
