Absorption and reflection | Waves | Middle school physics | Khan Academy

I'm showing you this beautiful picture of snow-capped mountains overlooking this alpine lake because there's a lot of light moving around.

Now we're going to talk about the different ways that light can interact with different media. But what I'm talking about can generally be true of waves. But focusing on light for a second, we've already talked about how if we go from one medium to another, if we're transmitting from one medium to another, that the light can go through a medium and as it goes through that boundary from one medium to another, its direction can change.

And we talk about that in another video and we call that refraction. We have refraction when we enter the material and then we have more refraction when we get out. Now there's other things that light can also do, and that's the focus of this video: reflection and absorption.

Now you probably have a sense of what happens with reflection. We can see a reflection of the mountains in the lake right over here, and the reason why we can see the reflection of the mountains in the lake here is because light that is coming from this mountain is hitting the lake and then it is bouncing off of it and then coming to an observer's eyeball right over here.

And so they see the light that's coming from here; they see it as coming from over here because it is bounced off. And so reflection is exactly that: light is coming in, it hits that other material, and then if it bounces off, that's reflection.

Now the other thing that light can do is get absorbed. Absorbed is when the material doesn't reflect any and doesn't transmit any through it if it's completely absorbing. And so that would be a situation like this.

Now the reality in the real world is we have oftentimes a little bit of all of the above happening. For example, when we look at this white snow up here, this is reflecting pretty well. So there's light that's coming from the sun, or maybe it's being reflected off the clouds, or maybe it's getting through the clouds, and when it hits that snow, it gets reflected.

Now the reason why we don't see a reflection the way that we see in this lake right over here is the snow reflects it in all different directions. But the fact that it's that bright color— in fact, you might need sunglasses to look at the snow just as much as you need to look at the sky— is that it's reflecting most of the light.

But if you go down here where we see the trees, the same light from the sky is hitting it, but not as much light is coming back to our eye. And that's because this part of the mountain— it might be trees, it might be rock, it might be dirt— is absorbing more of the light. But it's still reflecting some; we can still see it a little bit.

So this might be where most of it is getting absorbed, but a little bit of the light gets reflected, which we can see right over there. And if we think about the water right over here, some of the light is probably making it through and probably refracting as it does so. Some of it is getting absorbed as it makes its way through the water.

If this was a really deep lake, when you get to the bottom it could be very, very dark. And then, as we talked about, it looks like a good bit is reflecting. When we look at the image that we see in the lake, it looks almost as bright as the real thing.

So as I said, this is happening all around you. In fact, when people make fancy computer graphics, they actually try to do exactly what the light would do in the real world to make an image for your eyes that looks like the real world.

And they're thinking exactly about this: how much gets through the substance, how much gets reflected, and how much gets absorbed. And as I mentioned, it's not just light waves that do this; waves in general can do all of the above.