Hello, everyone, and welcome back. In previous chapters, we learned a lot about mechanical waves. The classic idea we used over and over again was flicking a wave on a string up and down to create a disturbance. Now, we've also learned in recent chapters a lot about electromagnetism. And in this video, I'm going to put those ideas together to show you a new type of wave you'll need to know called an electromagnetic or sometimes called an EM wave for short. I'm going to show you some key similarities and differences between these types of waves so that you can solve problems. Alright. So let's jump right in.

So remember that the whole idea behind a wave is that it is a traveling disturbance through space. With our wave on a string, you flick it up and down, and that creates an oscillation. That's a disturbance and it travels through space. So the big question here is what exactly is the disturbance for this new electromagnetic wave? What's the thing that's sort of going up and down? Like, the string was moving up and down. Well, as the name implies, the electro part in electromagnetic means that it's an oscillating electric field. And likewise, the magnetic part of that word means there's also going to be an oscillating magnetic field. Alright? So that's the big idea. You have an oscillating electric and magnetic field. I'm going to go ahead and show you what that looks like here.

With a mechanical wave, you just flick it up and down like this. What does it look like for an electromagnetic wave? Well, the first thing you'll notice is that you'll need a sort of 3D diagram to do this, and I'll show you why in just a second. So basically, it actually looks very similar. It's going to go up and down and up like this. So what's the thing that's moving up and down? Well, it's actually just the strength, the magnitude, and the direction of the field itself. So basically, what happens is that the fields oscillate by constantly changing magnitude and direction. What that means here is that on this wave, the electric field at this point points up, and then it gets stronger over here, then it gets a little bit weaker, and then eventually it actually just reverses and it points downwards. And then the cycle repeats itself over and over again forever. So that's what's going on here. That's what's oscillating the strength and the direction of the field itself.

Now, let's draw the magnetic field. Alright. So that was E over here. Let's draw what the B field looks like. Now, the first thing you need to know is that these electric and magnetic fields are always going to be perpendicular. This actually comes from the right-hand rule. Remember, a changing electric field will produce a magnetic field. So basically, what this looks like here is it's going to look like perpendicular to up and down is going to be back and forth. Alright? So this is going to sort of look like this. It's going to look a little funny because of the perspective. It's not a diagonal wave, but instead of going up and down, this wave is actually going forwards and backwards. Alright? And I can show you that using these arrows to kind of make it a little bit more visually apparent, that these things are actually going to be perpendicular. Alright? So basically, what this means here is that at any point, these arrows always have a right angle in between them. One's on the y-axis, and then one is on the z-axis moving back and forth. Alright? So these fields are always perpendicular, and that's basically what an electromagnetic wave looks like. You have an E field going up and down and a B field that's moving sort of forwards and backwards. Alright?

So the first thing we need to do is actually talk about the direction of these electromagnetic waves. So let's talk about the mechanical waves. Remember that mechanical waves, the direction was always perpendicular to the oscillation. The oscillation was up and down, that's where you flick the wave, the string up and down. But the direction was always sort of to the side like this. So that means that those were perpendicular. Well, for electromagnetic waves, it's very similar. The electro, for the direction, it's going to be perpendicular, but actually, it's going to be perpendicular to both of the oscillations. What that means is that this wave here is going to travel in a direction that's perpendicular to both the up and down oscillation of the electric field, and also the forwards and backward motion of the magnetic field. And the only two possibilities for perpendicular to both of those things is either the wave is moving to the left or to the right. So which one is it? Is it left or is it right? Well, like anything in electromagnetism, to find a direction you have to use the right-hand rule, and it's no different here. We use the right-hand rule over and over again and there's one for figuring out the direction of electromagnetic waves.

So to determine the wave direction, here's what you're always going to do. You're going to take your fingers, and remember, it's always best to do this yourself, and you're going to point them along the electric field. So for example, in this region over here, the electric field points up. And then what you're going to do is you're going to curl your fingers towards the magnetic field, so you're going to curl towards B, and then your thumb should point in the direction of travel. Alright? And if you do this, what happens is, you're going to take your fingers, point them up, curl them towards you, and your thumb should be pointing to the right. So what that means is that for this electromagnetic wave here, the wave is actually traveling to the right. This is the correct direction and it's not traveling to the left. Alright? You can always use the right-hand rule to figure out any one of these directions here. Alright? That's basically the introduction to an electromagnetic wave. Let's go ahead and take a look at another example problem.