Is reflection always necessary for objects to be coloured? When things glow and emit light due to excitation of electrons, is it just the same as stars produce light?

Question

Is reflection always necessary for objects to be coloured? When things glow and emit light due to excitation of electrons, is it just the same as stars produce light?

In: Physics

Answer 1

A difficult question to ELI5 given that you’re asking about some degree level science, but let’s have a go!
To put it simply, no reflection is not needed for an object to appear coloured *if that object emits its own light*.
A brick, for example, doesn’t emit light. The reason you see it as red is because white light (which is made up of the whole rainbow of visible light colours) hits it and every colour except the red is absorbed, with the red being reflected into your eyes. In that case, reflection is obviously necessary.
In the case of something that emits light due to excitation and the subsequent relaxing of electrons, however, there is no typical “reflection” of light happening. Instead in this case, a photon (which is like a tiny little pocket of energy) is absorbed by an electron, causing it to jump from one energy level to another (think like jumping up from one rung of a ladder to the next). Not every photon will excite every electron. It will only work if the photon contains _exactly_ the right amount of energy to move the electron one rung up the ladder – no more, no less.
The electron will, at some point, fall back down to the lower rung because electrons are pretty chill and are happiest at their lowest energy state (closer to the bottom of the ladder). When they fall back down, they have to get rid of the energy they absorbed before (because energy can’t just disappear!). The energy is emitted as a photon. Now, this photon will have a specific energy (the energy will be exactly the difference in energy between the two rungs of the ladder – incidentally, the photon coming out now will have exactly the same energy as the photon coming in!).
The energy of the photon will determine it’s properties. Photons with super high energies won’t be visible, and photons with super low energies also won’t be visible. Photons in the middle, though, may well have an energy that corresponds to a colour of visible light. In which case you will see a colour!