I think this question would make a bit more sense if we consider an ideal thought experiment. Imagine a molecule, call it Z, with just three electronic states. One molecular orbital has energy comparable to visible light and the remaining two molecular orbitals have an energy gap between them comparable to the energy of UV light.

Imagine now a surface that consists of just a couple of Z molecules, perhaps 20 or so. Now say that the surface of Z molecules is being bombarded with the whole visible light spectrum. Photon A strikes Molecule A and excites one of it's electrons to the next highest MO. The rest of the light is transmitted by that molecule and it gives off its characteristic color.

This of course happens to the remaining 19 Z molecules until all electrons are in excited states and now visible light is no longer sufficient in energy to cause electronic transitions.

My question is, what exactly happens now? Do all of the electrons remain in their excited states and hence all Z molecules transmit the entire visible spectrum, thus losing their unique color and appearing as white? Or do the electrons spontaneously fall back down to the ground state and begin the cycle once more? If they do fall back down to the ground state, do they re-emit photons of sufficient energy to do so?

Generalizing this to our realistic macroscopic world, why doesn't the same thing happen to colored objects in real life? Will the molecules that make my shirt red start transmitting only white light if I bombard it with visible light for a long enough period of time?

I do not have a very rigorous knowledge of quantum mechanics, mind you, so if I am missing something very obvious please point it out.


1 Answer 1


When left in an excited state, they can undergo a process called spontaneous emission, whereby they drop to the lower energy state an emit a photon corresponding to the lost energy. Roughly speaking, the time it takes for spontaneous emission to occur goes inversely with the size of the energy gap between states.

In fact, you don't even need to wait around for spontaneous emission to occur. Just by sending in another photon at the same energy as the transition, you can cause stimulated emission to occur. In either case, the de-excited electron emits a photon of the proper energy.

In real materials, the same principles hold, but it turns out that there are many more ways for an electron to release energy than by simply emitting a photon. An easy and important example is the phonon, which is a quantized vibration of the material. Phonons are essentially responsible for what we think of as heat. So your red shirt absorbs all light except for red light (which is reflected). The reason the absorbed light doesn't re-emit in a coherent way and mix with the red light to re-form the original white light is that most of those absorbed photons are actually lost to phonon emissions, and contribute instead to the heating up of your shirt.


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