The color white

Question

Is it supposed to be a frequency of light or a combination of all colors or both? What does it mean when a prism splits up a color?

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Answer 1

White light is made up of many different frequencies of light. Some other colours are also made up of a mix of frequencies – magenta, for example. Others (yellow, for example) might be, or might not be.

When light enters glass at an angle, it changes direction. That’s because the speed of light in glass is slower than in air.

Different frequencies of visible light have slightly different speeds in glass, so their direction changes differently – that’s how a prism “splits” light into different frequencies – the higher frequency “blue” component is bent more than the green or red, so a thin beam of white light gets split into a rainbow of colours.

Colours are how we *perceive* the different mixtures of frequencies of light, but our colour sense is pretty limited. Light has a full continuum of possible frequencies, but our eyes lump them into three bands we perceive as “red”, “green” and “blue”, which each stimulate diffferent cells in our retina.

Some light stimulates just the “red” cells, and will look red. A slightly higher frequency will stimulate red AND green cells, and we see that as yellow. Higher still, and the red cells aren’t stimulated any more, and the light looks green.

So it’s possible to have “yellow light” which is a single, pure frequency, and a different “yellow light” that’s actually a mix of green and red, that looks exactly the same because it stimulates the red and green cells exactly the same way as the yellow light – though a colourblind person might see a difference.

There are also “blue cells” in our retina that respond to even higher frequencies of light.

If we stimulate all three types of cells equally, all at once, we perceive that as “white”, but there’s no single frequency of light that does that. Sunlight has a pretty even mix of all frequencies, so it seems white to us. A computer or phone screen will generally just mix three specific frequencies (red, green and blue) to produce white.