# Where does visible light come from? [closed]

I think light begins in subatomic physical processes. Is that correct?

• All wavelengths exist. Your model is too simple because it assumes that only one wavelength gets bounced back to you from the wall. In fact, you get a mixture of infinitely many wavelengths. Your eye averages these three times, first putting more weight on the wavelengths near 440, then (I think) 550, then (I think) 580 --- maybe these numbers are a bit off. Those three averages are sent to your brain, which converts those three numbers to a perceived color. Many different mixtures of wavelengths can yield the same three averages and hence the same perceived color. Oct 4 '17 at 0:49
• @WillO That looks like it should be an answer Oct 4 '17 at 1:07
• @DavidZ: I thought that it should not be an answer because it did not address the main question about photons interacting with electrons and changes in energy levels. Oct 4 '17 at 1:12
• @WillO Ah, I suppose you're right - I guess I was a little hasty in reading. Well, given that the question is asking so many separate things I've put it on hold for now. Pedroski, I'd recommend narrowing the question down to clearly ask one specific thing, along the lines of what Floris answered. Oct 4 '17 at 1:20
• @DavidZ: Ah. I'd just reconsidered and posted as an answer when I came back to find your "I suppose you're right". I guess this was a close call. Oct 4 '17 at 1:22

There are at least two completely different mechanisms that can lead to "redness" of an object.

Most commonly, your material has a pigment that absorbs certain wavelengths more strongly than others. Most paints are in this category. You make a surface that has a degree of internal scattering of light, to increase the mean path of the light through the pigment before it comes back at you - and if you remove the blue and green components you end up with red scattered light. This works best when the illumination contains "all wavelengths of visible light". Read about CRI (color rendering index) to see what happens when you are missing certain wavelengths in the illumination (a problem with many non-incandescent light sources).

The second is more like the mechanism you describe - where electrons are excited through absorption of photons, and they emit very specific wavelengths during their decay. All fluorescence falls in this category - you irradiate with one wavelength, and light from a different wavelength is returned to you.

Where the first mechanism requires "red" to be in the illumination, the second mechanism allows an object to look, for example, "yellow" when you illuminate it with "green". See for example this recent answer I wrote about ping pong balls.

All wavelengths exist.

Your model is too simple because it assumes that only one wavelength gets bounced back to you from the wall.

In fact, you get a mixture of infinitely many wavelengths. Your eye averages these three times, first putting more weight on the wavelengths near 440, then (I think) 550, then (I think) 580 --- maybe these numbers are a bit off.

Those three averages are sent to your brain, which converts those three numbers to a perceived color. Many different mixtures of wavelengths can yield the same three averages and hence the same perceived color.