|MadSci Network: Physics|
If you shone pure yellow light (ca. 560 nm) through a prism onto a screen, you would see a single band of yellow light. If, on the other hand, you shone a mixture of pure red (ca. 750 nm) and pure green (ca. 510 nm) in the correct proportions to look "yellow" to the human eye, you would see two bands corresponding to the two original sources. Similarly, if you simply filtered out the lower wavelength light (<490 nm), you would have a "yellow" source that would give a complete red through green spectrum of bands on the screen. The source of the differences is not in the physics of electromagnetic radiation, but in the anatomy of the human eye.
The retina is composed of two sets of photoreceptors: rods, which absorb all wavelenths of visible light; and cones, which only absorb specific subsets of visible light. There are three sets of cones: red, green, and blue. Each cone responds to a spectrum of frequencies centered at the chief color for the cell and overlapping the adjacent spectra of the other types of cones. For example, the green cones respond to light between red and blue, and the red cones respond to light from red through green, so that yellow light will activate both the red and green cones simultaneously. The retina then "sums" the red and green cones' signals to send the signal "yellow" to the brain. This makes the system easy to fool, since any mixed light that stimulates the red and green cones in the right proportions will generate a "yellow" signal.
So, getting back to your question, true yellow light is not made of green and red light, however there are many mixtures of different wavelength lights that our eyes perceive as "yellow". As such, the spectrum projected onto the screen will depend entirely on the form of "yellow" light shining through the prism. Two examples would be a yellow laser beam, which would give a sharp yellow band, versus light from a yellow computer screen, which would give two sharp red and green bands.
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