So, the idea is this a changing E generates a B. And, a changing B generates an E. I change B; I can generate E. But if I change E, I can generate B. And maybe, these things can balance out just right that you can generate electromagnetic waves. And, that is indeed what happens, okay? An electromagnetic wave looks like this: I've got an E field that is going up and down like so. And, this means that it's pointing up in this region and then it's pointing down in this region, and so forth: up, go through zero, pointing down and just keeps oscillating back and forth. But, that changing a field can generate a changing B field. And, the B field is orthogonal to that. It's at a 90-degree angle to it. So if E is up-and-down in the plane of the board, then B is in fact going in and out of the board. And this is a little hard to draw, but the idea is B is pointing this way, and then it's pointing into the board, and then it's pointing out of the board, and then into the board and so on. Okay? So, these things are doing something like this: E field is going up and down. B field is going in and out and so they're sort of doing this: Okay? It's like two fish swimming along one going up and down one going left and right. As this happens this entire thing propagates to the right. So, what is weird about this? What's weird about this is there's no charges anymore. There's no wires of current anymore. These things can just propagate on their own because the changing E field, creates a B field changing B field creates a D field. And, they just keep going like that -- sloshing back and forth. And, this is called an electromagnetic wave. So, how fast did these things propagate? We said that it's gonna propagate to the right. It is called a transverse wave because the E field and the B field are perpendicular to the propagation direction. Okay? So even though the wave is moving this way, the E field is oscillating up and down. The B field is oscillating in and out those are both perpendicular to the propagation direction. How fast are these things moving? How fast does an electromagnetic wave propagate? Well, I think we know the answer to that, right? They propagate at the speed of light 3 times 10 to the 8th meters per second. So, This stuff, electromagnetic waves, came about from something called Maxwell's equations. And, up until Maxwell, we had independent quantities. We had electricity. And then, we had magnetism. Electricity was governed by things like Coulomb's law. Magnetism was governed by things called AmpÃ¨re's law. Faraday's law started to tie those two together, but it wasn't until Maxwell came along and combined all of those things into one quantity, that we ended up knowing that light itself is made up of electromagnetic waves People knew a whole bunch about light. People knew a whole bunch about electricity and magnetism. But, what they didn't know was that light itself was electromagnetic waves. And, Maxwell put all those equations together -- combined them into four neat equations. And if you happen to continue in physics and take my upper-division electricity and magnetism course, it's all about Maxwell's equations. Start with those four equations, and spend an entire year trying to uncover all their interesting phenomena. Okay, it's really very fascinating stuff. All right. If we have these changing electromagnetic waves that are flying around the universe how do we detect them? Well, let's think about this wave right here.