Question

Coloured objects, red ones, blue ones, whatever. For this purpose, one assumes that light is waveforms as opposed to particles of energy. Traditional theories state that a green object appears green because it reflects green light and absorbs other frequencies: red, likewise. What property does the substance have to cause it to reflect certain wavelengths and to absorb others? Is this related to luminescence and fluorescence?

Answer 1

The biggest effect is actually what colour something absorbs. Different colours of light have different energies. The bluer the light, and light comes in blobs called photons, the more energy the photon has.

The electrons inside atoms can only have certain energies so they have what are called energy levels. Maybe they can absorb a certain amount of energy or twice that amount or two and a half times that amount, changing in discrete amounts. They move up and down energy 'shells', between orbitals and energy levels. A substance can only absorb light if the difference between one energy level and the next is equal to the energy of the photon. It will gain the right amount of energy to absorb that photon of light.

This is also is why substances have that characteristic spectral fingerprint, an absorption pattern of certain frequencies of light that they absorb and certain lights that they reflect.

Fluorescence is a related effect. This happens when you get high energy photons, for example ultraviolet light, which hit an atom. The atom will absorb that energy and then instead of releasing it all in one big lump it releases it in two or three smaller lumps which will be a different colour to the UV, a lower frequencies that you can see. Something can absorb ultraviolet light then emit blue light or green light and it looks like it