I already know that color is the result of electromagnetic waves bouncing off a surface into my eyes and my brain interpreting that signal. But what causes a surface to reflect one portion of visible electromagnetic waves but not the others?
Photons have the energy of hf… simply a constant times the frequency… so each frequeny of light has a specific energy in each photon.
Each electron can orbit the nucleus of the atom at various heights, or energy levels.
Then, each electron can
- absorb a photon and move up to a higher energy level, the difference in energy coming from the photon
- emit a photon as it drops to a lower energy level… the difference energy level being the energy going into the photon.
Mostly the atoms at room temperature are absorbing of visible light, and not doing much emitting of visible light…But they will emit infra-red for example, which explains why the absorbing isn’t making the material hotter and hotter…
Now the same atom can have different colours depending on how its bonded with its neighbouring atoms… the bonding affects the energy levels that the electrons are liable to have and the energy level changes they can do. (which is beyond me … but thats why it changes for different substances , its not a colour fixed for each element… )
Interference is another reason for objects having colour - in cases such as soap bubbles, lens coatings, butterfly wings etc - incoming light reflecting off two closely parallel surfaces (i.e. the inner and outer surfaces of the bubble), at least when thought of as a classical wave, destroy or reinforce each other at certain wavelengths - the wavelengths that survive (or are amplified) are the colours we see.
To add to lsilder’s post, a lot of dyes, for example, are organic molecules which have extended chains of double and single carbon bonds. These bonds are made up of a long cloud of electrons, and when this cloud is the right size, the gap between its energy levels just happens to fall within the narrow band of the electromagnetic spectrum that we can see.
So the energy absorbed by these electrons is visible light, hence we perceive colour.
These structures are called chromophores. See here: Chromophore - Wikipedia
Also note that organic chromophores tend to be fairly reactive, so they can get broken down when they absorb energy. If the chromophore structure is broken, then it will no longer absorb within the visible light range, and this is what causes dyes to fade over time.
Bonus fact: In some such molecules, the chromophore will actually change shape when it absorbs light, and this is the mechanism by which the light sensors in our eyes work.
perhaps we can rephrase the OP: If you knew enough about an object’s chemistry and shape, could you figure out its color without looking at it?
Chemistry and shape, including the electron bonding configuration, and a lot of quantum mechanics and a really powerful computer and a generic “you” (i.e., not me)… I’d say yes.
Oh, and the intensity and spectrum of light hitting it, and the eye’s spectral response…
It’s going to be a lot easier for some substances or objects than for others. Colors due to diffractive effects are pretty easy to calculate: You just need to know the sizes of the relevant structures. Colors due to molecular energy levels, well, that depends on the complexity of the molecule. You could probably calculate that chlorine is green by hand, as a single homework problem, but some big fat organic molecule would require a computer.
Why can mirrors reflect any color, unlike most other surfaces?
Mirrors reflect all colors because they don’t absorb any visible light (mostly; there’s always a little absorption). The outermost electrons in metals are conducting electrons, which means they are shared among all the atoms, instead of being bound to one or two atoms. These electrons are able to respond fast enough to prevent visible light from penetrating much into the metal, and (almost) all the energy is reflected.
If you could see at even higher frequencies, like UV, the mirrors would “color” the reflected light, because some of the UV wavelengths would be absorbed.