Electromagnetic waves and matter

0:00- [Instructor] Look at this apple. What allows you to see it? Well, you probably know it's light, but is that light produced by the apple or it's coming from somewhere else? And take a look at this picture, what's happening over here? Well, here we have pencil immersed in a glass of water, which means you can see through glass and water.

0:17So what's happening to light over here? And why does this pencil appear so bent? What's going on? How can we use the model of light that we already know to answer these questions? That's what we're gonna figure out in this video. So let's begin. We already understand the wave model of light.

0:32If you take a charge and you just start wiggling it, the disturbance propagates outwards through the electromagnetic field, kind of like a ripple in a pond, and we call this the electromagnetic waves. And it has all the different properties of waves like veiling, frequency, amplitude, and speed, and so on.

0:47But for our purposes, let's concentrate on the direction of the light. So again, we know that if you have a source of light, it produces electromagnetic waves like the ripples in a pond. But in what direction is the light traveling? Well, the light's traveling in all the direction.

1:04It's traveling forward, it's traveling this way, and that way, and sidewards, in all the direction in this particular case. So let's use arrow marks to represent that. Okay, let's do this one more time and here we go. So the ara marks nicely represent the direction in which the light is traveling.

1:22So guess what? Here's another model of light. We can use rays to represent the direction in which the light is traveling, we call this the ray model. And in this model we see that light travels in straight lines. Amazing, isn't it? But does it always travel in straight lines?

1:38Let's see. What happens when you shine light into a mirror? Well, let's again use the ray model. We have rays of light coming out from this torch, for example. And let's see what happens when it hits the mirror. It bounces off and therefore you can clearly see light is no longer traveling in straight lines.

1:53And this has some interesting result. So what happens when this rays of light enter into our eyes? Well, our brains don't understand that the light bounced off. Our brains always think that light's traveling in straight lines. So our brains think that these two rays of light, for example, they came from somewhere over here, and that's the reason why we see an image of that light inside the mirror.

2:16This is why we see images inside the mirror. We call this phenomena reflection of light. And that's why we say that we are seeing reflection in our mirror, right? And so whenever light bounces off of something, it can be anything, not just mirror, we call this phenomena reflection.

2:31And that's how you see an apple, for example. An apple cannot produce its own light, and that's why if the room is dark, you can't see an apple. But when you switch on the light, you can see the apple, because the light is being reflected off of the apple and it's entering into your eyes.

2:45That's how you see it. But wait a second. If we are using white light, why does the apple look red in color? Shouldn't the apple also look white? Woo, that's an interesting question. If you zoom in, what's really happening is this. What we see as white light is actually a combination of all the colors of a rainbow.

3:02So all the colors of the rainbow are falling on the apple. But guess what? The apple does not reflect all the colors of light. The apple only reflects red color. All the other colors are absorbed by the apple. We call that absorption, okay? So light not only gets reflective off of things, but they also get absorbed.

3:24And what happens to the energy of that light? It doesn't just disappear, but that energy gets transferred to thermal energy, and that's what heats up the apple just a little bit. But anyways, this is the reason why we see the apple to be pretty much red in color, because it reflects red light and it absorbs the rest.

3:42Similarly, the leaf appears green because it reflects green light and it absorbs the rest. Now, whether a particular frequency of light gets absorbed or reflected depends on the frequency, and it depends on the material itself. The skin of the apple interacts differently with light compared to the skin of the leaf.

4:01Okay, but what else can light do besides reflection and absorption? Well, guess what? Light can also get transmitted. And you can see that in glass, for example, of course there is some light that's being absorbed, some light is also getting reflected, which we can't see, but you can see a lot of light is actually going through.

4:18We call this transmission. And again, just like before, whether light is gonna get transmitted, reflected, or absorbed, depends on the frequency of light, and it depends upon material. It turns out that the frequency of light that's in the visible range, well most of that light gets transmitted through glass, but not through apple, for example.

4:38And when light does get transmitted, you can see that it again does not go straight, it bends. You can clearly see that over here, right? And this causes some pretty cool effects. For example, if you were to insert a stick inside say water, again, visible light can transmit through water.

4:55And when light comes out of water, and say, reaches our eyes, look, it bends. But again, just like before, our brains think that the light is always coming straight, and so we will see the light coming from somewhere over here. So we will see the tip of this stick, not over here, but over here, and that's why it appears bent to us.

5:16And the same thing's happening over here. Because light bends as it comes out of glass and water, we see the image to be pretty distorted and it looks pretty cool, huh? And by the way, because of this particular bending of light as light transmits through different medium, we can create things like telescopes and microscopes.

5:35And the main components of these devices are those lenses. And the last important thing about transmission is, the amount of bending that happens, again depends upon the material and the frequency of light. Different frequencies of light bend to a different degree.

5:53And this is the reason why when you shine light through a prism, you get the spectrum, you see the rainbow colors of the light. Because different frequencies of light, different colors, bend different amounts, and so the colors separate out. And if you're wondering, hey, is this how we get rainbows?

6:10The answer is yes. Rain drops act like prisms, and they separate the white light from the sun giving us the beautiful rainbows that we see. Finally, remember that visible light is just a small portion of the entire electromagnetic spectrum. And whatever we discussed about absorption, reflections, and transmission, applies to the entire spectrum.

6:31And so just to give you an example of how cool this is, if you were to shine visible light on your hand, then some of it gets reflected because of which you can see it, the rest of it'll get absorbed, but nothing gets transmitted through. And that's why if you were to put a screen behind, you will see the shadow of your hand because there's no light transmitting through your hand.

6:49However, instead of visible light, what if you used X-rays? Well, turns out that a lot of X-rays can pass through your skin, it can transmit through your skin and through the soft tissues, but a lot of it will be absorbed by the bones. So what will the shadow look like now?

7:06Well now you'll get a shadow of your bones because it's your bones that are absorbing the X-rays and your skin and the tissues are transmitting it through. And this is the reason why X-rays are used for imaging your bones. Isn't that wonderful? That's amazing, isn't it?

7:21So long story short, when you shine any light on any object, it can be reflected, absorbed, or transmitted. And how much does an object reflect or absorb or transmit light purely depends upon the frequency of the light and the material.