Does it have to do with density? Also how do reflections work in this sense?
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Not an easy question.
Light is an electromagnetic wave, and it interacts with electrically charged particles (such as electrons).
When an electron absorbs light, it absorbs its energy, and the energy of the electron increases.
Now we need some quantum mechanics: one of the most important ideas behind quantum mechanics is that most things have quantized energy states – e.g light is not just a wave, but it can be also seen as a number of distinct particles (“photons”), which have energy depending on the frequency of the light (higher frequency = more energy per photon).
Electrons cant just absorb some of the energy of the photon – they either absorb the entire photon, with all its energy, or they leave it alone.
Now, electrons (in solids) can only have certain states. They can only have certain values of energy, depending on how they are constrained by their surrounding particles. The reason for this is that electrons store energy as oscillations, and they can only oscillate in certain resonant frequencies – you can imagine that a bit like a guitar string, which always has to vibrate at a certain frequency, no matter how you strike it.
This means that if a photon hits an electron, and the energy of the photon would increase the energy of the electron, but the electron does not have a possible state at that new level of energy, the photon passes right through and is not absorbed. In other words, the photons energy must fit one of the possible states of the electron to be absorbed.
Remember that the energy of a photon depends on wavelength. Thats the reason why transparent materials are transparent for some wavelengths, but absorb other wavelengths. The glass we use for windows, for example, is translucent for visible wavelengths, but absorbs UV. Very useful, because it means you will not get sunburns jnside your house or car.
**TLDR: photons have a specific energy. Electrons in solids can only have certain values of energy. If energy of photon does not fit one of those values, it can not be absorbed.**
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