What makes stuff have a specific colour?

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I know what colour “is”, how it’s perceived and all, but I’m wondering what makes stuff “have” a specific colour. Like why, for example, an Iris flower has this purple color, while a rose is red.

In: Physics

There are two things at work in most colours found in nature.

The first is pigment ( https://en.m.wikipedia.org/wiki/Pigment )

Plants have evolved to produce chemical substances that reflect light in a particular way.

The other phenomenon is structural coloration ( https://en.m.wikipedia.org/wiki/Structural_coloration )

This is the phenomenon of animals or sometimes plants producing a specific pattern at the microscopic level which reflects light in a specific way.

Any material has the property to absorb light at different wavelengths differently. So a red rose just absorbs most light of the visible spectrum so only the red ends up in your eye.
Coca cola is near black, so it absorbs most visible wavelengths. But if you use infrared, coke is clear because none of its contents absorb or reflect infrared very much. X-ray (which is just a kind of “light” way out of the visible spectrum) passes through most materials but less so at metals. So it can be used to look into a suitcase and find metal objects.

Everything is made up of atoms and atoms are surrounded by electrons.

Electrons exist at certain energy levels. They usually want to be at the lowest energy level possible, but this “lowest energy level” possible determines on the presence of other electrons. Only two electrons can share the same energy level for a given atom. More than that, and those other electrons have to occupy higher energy levels.

The funny thing about these energy levels is that they are discrete. They can’t just be any ol’ thing. For example, let’s say quantify one energy level as a “1” and the next higher up energy level as a “2”. Electrons can only exist at those discrete energy levels. They can’t exist, say, half way between them. If an electron received enough energy to move to the next energy level, it would instantly jump to the next one; it would never be at a place in between. More to the point here, the electron would *only* jump to the next level if it was supplied enough energy to move completely to the next level. In sufficient energy is ignored.

Light has energy and that energy is determined by its wavelength (e.g. color). So if a photon of life hits an electron and has enough energy to cause the electron to jump energy levels, it jumps up. But then it immediately jumps back down (because it wants to be at the lowest energy level possible) and, in doing so, releases that energy in the form of a photon. The specific kind of photon depends on the energy level it is coming from and the energy level it is landing on.

If the photon does not have the right energy/wavelength, then it is ignored by the electrons of those atoms and simply passes through.

In short, the specific configuration of electrons in the atoms of a substance determine what colors of light pass through, are absorbed, and are emitted by the object.