Not sure if I’m saying this right but what exactly makes some mass absorb light and other mass reflect it? Or let it pass through completely?
In: Physics
Light doesn’t, by default, interact with matter. When light enters a material, electrons in that material move. They act like little balls on springs, so the way that they move depends a lot on the spring (and since they can move together, the way that they clump and drag atoms with them also matters).
If the electrons move freely, then the light usually reflects. This is why metals are reflective. If the electrons’ movement is small unimpeded then the light passes through but slows down. It also bends which can create a white appearance.
If the electrons’ movement is small and impeded, or really large, then the light stops as the electrons take its energy and pass it to nearby atoms to produce heat.
Now, the movement of the electrons does depend on the frequency. Just like how it’s easier to shake a car or push a swing at the right frequency, these wobbly electrons respond differently to different frequencies of light.
Materials can only absorb/emit certain amounts of energy per photon. So if a photon doesn’t have the right amount of energy to be absorbed it passes right through.
For example, the electrons in diamond or glass are bound so tightly they aren’t allowed to move with visible light and therefore can’t accept that energy making them transparent. On the other hand, electrons are free to move around in metals and therefore they absorb and reflect most visible light.
Interestingly, things like clothes mostly transmit the colors we see them as rather than reflecting them. However, because there are so many tiny fibers and they’re oriented randomly, the light is transmitted and deflected in every direction equally including back the way it came. I like to picture it as a pinball machine where each photon is like a pinball that flies in fast and hits so many parts it can come back out in any direction. This gives it the matte look.
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