So, objects are certain colors because they are absorbing every color except for that one. So why is it that if I take a projector and project a blue image on a red wall, the red wall still reflects the blue image rather than absorbing the blue light? I don't know if this is considered a dumb question, but I was thinking about it and it got me curious (also, I haven't taken any kinds of physics).
[Physics] How do projectors work
everyday-lifevisible-light
Related Solutions
Thus is a very common confusion, and it occurs because light is neither a wave nor a particle but instead it's (currently best described as) a quantum field. The wave and particle descriptions are approximations that apply under some circumstances. In particular the photon model is a good way to describe how the electromagnetic field exchanges energy with it's environment. When the light transfers energy to something else the energy transferred is an integral number of photon energies.
So, in your example, glass is clear because for visible light there aren't any energy levels spaced one photon energy apart. Since light can only interact with the glass by exchanging energy in photon sized lumps the interaction can't occur. Glass does absorb in the ultraviolet because photon energy is proportional to the light frequency and at uv frequencies the photon energy is big enough to excite electrons in the glass.
The green pigment in the paint on your wall has been chosen to have electronic excitations that correspond to the photon energy of red and blue light, but none with an energy matching green light. This means that red and blue light falling on the wall is absorbed but green light is reflected. In general with solids, when a light excites an electronic transition and is absorbed, the energy of the excited electrons is dissipated as lattice vibrations. Only in some circumstances is it re-emitted as light, in which case you get fluorescence or phosphorescence. So it isn't the case that light is absorbed and re-emitted as green light. The green light is reflected and stays green, while the other colours are absorbed and their energy ends up heating the wall.
The green light reaching your retina has a photon energy that matches the optical pigments in the M-cones. Thus the light is absorbed (in photon sized chunks) by exciting electrons in the optical pigments.
Under normal circumstances, what you are seeing is the steady state condition where the rate of absorption is equal to the rate at which energy is conducted away or radiated away again, so the material doesn't heat up. As I understand, unless a material fluoresces, the de-excitation happens in the infrared.
Also, you should realize that just because a photon being absorbed means that an electron went from energy $E_1$ to energy $E_{10}$, there is no reason to believe that in de-excitation, the electron will go back from $E_{10}$ to $E_1$ again in one jump. The energy levels of a solid are very complicated (band structure) and has very many finely spaced energy levels that are almost like a continuum. Where there are gaps in the band structure, the material is transparent.
Here is an example of the band structure of a random material (density of states of gold in the middle). I believe colour has more to do with the conduction bands (the upper squiggles) and the relative number of available states.
Related Question
- [Physics] Why are solar panels blue, rather than black, when black absorbs more light
- [Physics] Why do we see brown light
- [Physics] Blue light filtering
- [Physics] How come we can get mirror-like effects from dark surfaces
- [Physics] Why does an yellow object absorb blue instead of all colors other than yellow
- [Physics] What colors reflects the most infrared radiation
Best Answer
This is due to a psychological effect known as Color constancy. Your brain seems to see something based on the context around it, for instance the blue image projected on the red wall may be more black than blue but your brain will compensate for some of that because you also see the red wall.
This is also the source of a number of related optical illusions. For instance consider the image below taken from here. The squares labeled A and B are actually the same color, but one looks dark gray and one looks white because tile B seems to be in a shadow and surrounded by dark tiles and A is not so our brain perceives that B must be lighter.
Here's another optical illusion that may be closer to your question. The center piece in both images is the same color (gray) which you can see here.