Thought about this today and couldnt wrap my head around it. Could it have something to do with particle density?
If it has something to do with the abilty to absorb and reflect it, how does that exactly work?
Thanks in advance!
In: 6
You made me read my old university physics courses on this question, and made me realized I’ve forgotten 90% of it… Anyway, I’m still able to understand it when I read it, so I’ll try to explain.
Light is an electromagnetic wave. That means that light creates an electromagnetic field where it passes. An electromagnetic field will move electrons, ions, anything that has an electric charge. But since energy is conserved, if you move something, you have to use up energy. So if the electromagnetic field moves something, it progressively fades out. If light goes through something, it means that the electromagnetic field hasn’t been able to move anything in the material. If it doesn’t travel through it, then the electromagnetic field must have moved something there, and has disappeared in the process.
Now what can this electromagnetic field move ? Well, anything electrically charged. It can be electrons, ions… Let’s take electrons for example.
Electrons are bound to an atom. Normal light isn’t powerful enough to make them leave their atom, so they will move a bit, and then go back to their original place. Now, we said that light is an electromagnetic wave; we talked about the electromagnetic part, let’s talk about the wave part. Light being a wave means it has a frequency : it “vibrates” a certain number of times per second. For visible light, it’s about 100’000 billion times a second (yes, that’s a lot).
Take a little spring. Put some mass at one end, and hang the other end to your hand. Move your hand really slowly. The mass makes the exact same movement as you hand. Now move really quickly. The mass nearly doesn’t move. Finally, move not too fast, but not too slow. Now the mass is moving even more than your hand, and you sense a force exerted on your hand.
The electron is like the mass, and it is bound to its atom like your mass is to your hand. Vibrate too slowly, and it moves along without absorbing energy. Vibrate too fast, it doesn’t notice. Vibrate exactly at the right speed? Then it will vibrate too, and use up energy in the process.
The right speed depends on the atom, on the surrounding atoms (is it a liquid? a solid?), on lots of things.
The same idea can be applied for ions.
Metals are a special case, since they’re conductive. So there are electrons in them that are not bound to atoms : they will move anyway, not depending on the frequency. So any electromagnetic wave will move electrons and fade, hence why metals are never transparent.
I tried to make it the clearest possible, not sure I’ve managed to do it, and it’s definitely way too long, but I tried!
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