Electromagnetic waves | Physics | Khan Academy

What's common between a Wi-Fi router, our bodies, and an incandescent bulb? We all give out electromagnetic waves. But why do we do that? And why are they all so different? How do we use some of them for wireless communications? Let's answer all of them.

Let's start by asking ourselves: what exactly are electromagnetic waves? Electromagnetic waves consist of oscillating electric and magnetic fields. A changing electric field produces a changing magnetic field, which produces a changing electric field, and so on and so forth. You can see in this animation that the disturbance gets propagated. But the beautiful thing is that this disturbance can get propagated in a vacuum; it does not require any medium, and it carries energy. That's, for example, how we get energy from the Sun in the form of electromagnetic waves.

But how do you produce these electromagnetic waves, you ask? Well, just accelerate charges. Accelerating charges will change electric fields; you will change magnetic fields, and that's how electromagnetic waves will be generated. A cool way to accelerate charge would be just to take an electron and just wiggle it up and down. If you continuously wiggle an electron up and down, it will keep losing energy as electromagnetic waves. So, accelerated charges create electromagnetic waves.

If you consider the Wi-Fi antenna, for example, you'll find that there will be electrons oscillating up and down very quickly. Okay? And because electrons are charges oscillating up and down, they will generate electromagnetic waves, which we call radio waves. Similarly, if you consider your own bodies, you have electrons inside. Our bodies have charges inside that are vibrating. Just because of the temperature, they're vibrating over there, and that will again generate electromagnetic waves, which happen to be infrared radiation.

The same thing will happen inside an incandescent bulb. When the filament is heated, it heats to such a high temperature that the electrons are vibrating randomly. As a result, you have electromagnetic waves being generated, giving us light. But, of course, now comes the big question: why do we get radio waves over here, infrared over here, and light rays over here? What's the difference between them? In all cases, it's the charges that are going up and down, right? Then why do we get different kinds of electromagnetic waves? What's the difference between them?

To answer that question, we need to understand some key properties of these electromagnetic waves. Let's say this is our electromagnetic wave. Remember, it has both electric and magnetic fields. I'm not showing both of them. Imagine this is just the electric field, and it's moving. You can imagine the whole wave is moving to the right, for example.

Okay, and one of the key properties of the wave is how big the wave is—the height of the wave. Okay? That's what I mean. So that technically is called the amplitude of the wave. So this height is what we call the amplitude. For example, this would be a very small amplitude wave; this would be a very large amplitude wave. And of course, remember, when I say height, there's nothing physical over here; these are electric and magnetic field values.

Going back to the animation, it's more appropriate to think of amplitude as the maximum strength of the electric or the magnetic fields. Another important feature of a wave is what we call its phase. It's easier to understand what phase is if I were to look at just two specific points on a wave. So let's compare only these two points on the wave and see what happens to them.

I'll just only compare those two points and see what happens to them as the wave moves forward. I'm going to move the wave to the right, so notice they will start oscillating; they'll go up and down. But look, they are going up and down together. They are in sync with each other, so we say they are oscillating in phase with each other.

You can think of phase as kind of representing where they are in their cycles of oscillation. They're both in phase because they reach their valleys together, they reach the zero together, they reach the peak together, and so on and so forth. Any two points on the mountains are in phase; any two points on the valleys are in phase with each other. But what about these two points? Are they in phase with each other? Let's see.

Let's concentrate on them and move the wave and see what happens. Hey, no, they're not! What do you see? You notice that when one is going up, the other one is going down. So look, they are not oscillating together; they're not in phase with each other. In fact, we say they're out of phase. That's what we say. But what's important is that they're not in phase with each other.

Okay, this idea of phase will be super important later on; we'll come back to it later on. Another important feature of our wave is its wavelength. You can think of wavelength as the distance between any two consecutive peaks or any two consecutive valleys or, in fact, any two consecutive points which are in phase with each other. We saw which are in phase, right?

So, for example, if you consider the distance between these two consecutive peaks, this length is what we'll call the wavelength, and it is usually represented by the symbol lambda. Okay? So again, if you go back to our wave, this would represent short wavelengths; this would represent long wavelengths.

Okay? And one last important feature of our wave is its frequency. What is frequency, you ask? Well, take any point you want and then measure in one second how many waves go past that point. The number of waves passing through a point per second is what we call frequency. You can think of it as the number of waves passing through a point per second.

Okay? So the unit of the wavelength is meters because you're measuring distance. And then what is the unit of frequency? Well, it's number of waves per second, so the frequency would be numerically just a number. So, number per second would be second inverse. But we often call second inverse when we use it for frequency as Hertz.

So, for example, if I say frequency is 10 Hertz, it means from a given point, 10 waves are passing by that point per second. Now, here's the cool thing about wavelength and frequency: if you know them, if you know the wavelength and frequency, you can figure out the speed of the wave.

Let's see how. In fact, let's take some numbers directly to see how. So let's take some simple numbers: let's say we have a wave which has a wavelength of 2 m, and it has a frequency of 4 Hertz. How do we figure out the speed of this wave? Well, what we're going to do is let's imagine there's a wave that starts from here, and we'll just wait for one second.

Okay? So wait for one second. Now, in that one second, I know four waves are going to pass by. So let me just draw them. So I'm waves going to start from here; it's a peak. So four waves are going to pass by in one second. So here's wave number one. Peak to peak is one full wave. Okay? Wave number two, wave number three, and wave number four.

So in one second, this whole thing happened in one second. Now, if I figure out how much this total distance is, I'm done, because then that represents the total distance traveled by the wave in one second, and that is the speed.

So, I need to calculate what this distance is to figure out the speed. How do I do that? Well, I know what the wavelength of this wave is, right? It's given to me. So, can you pause and think about what this total distance is going to be, and as a result, think about what the correlation between the velocity or the speed of the wave, the frequency, and the wavelength would be? Why don't you pause and try?

Alright, okay! So since the wavelength is 2 m and I have four waves, the total distance traveled by the wave would be 2 * 4; it would be 8 m. So, in general, if the wavelength is lambda, this would be lambda, and if the frequency is f, there would be f-number of waves over here. That means the total distance traveled in 1 second would be f * lambda. Therefore, the speed of the wave would be f * lambda.

Now, looking at this equation, we might think that, hey, the velocity of a wave depends on the frequency, and the velocity of the wave depends on the wavelength, but it turns out things are a little bit more complicated; it depends on which wave we are dealing with.

So now, let's come back to electromagnetic waves. It turns out for electromagnetic waves in vacuum, the velocity is just a constant. Any electromagnetic wave you take in vacuum, it'll always travel at approximately 3 * 10^8 m/s. We often call this C—the speed of light in a vacuum. Okay? It is independent of the frequency of the wave; it is independent of the wavelength of the wave.

Which means for electromagnetic waves that are in vacuum, since this is a constant, we notice that that means if you have a high-frequency wave, it should have a lower wavelength. If it has a high wavelength, it should have a lower frequency.

So, for electromagnetic waves in vacuum, you can see that the frequency and the wavelength are inversely related to each other. And now we can go back to our original question: why are these things giving out different kinds of electromagnetic waves?

Well, because the charges over here are oscillating at different frequencies. In your Wi-Fi, the charges are oscillating at lower frequencies, so you get low-frequency electromagnetic waves, which we call radio waves. Inside our bodies, the charges are vibrating at slightly higher frequencies, and so now you get slightly higher frequency waves, which you call infrared rays. Inside the light bulb, it's even higher frequency, which we get called visible light because we can see it.

Just to give you a feeling for numbers, okay? So your radio waves—that's coming from your Wi-Fi—is in the order of about a billion Hertz, meaning the electrons over here are oscillating about a billion times per second. Okay? And when it comes to infrared, it becomes 10^13, and for visible light, it becomes 10^14. You can even have higher frequency waves which go into ultraviolet rays; you have x-rays, you have gamma rays, and so on.

You can also see as the frequency increases, the wavelength is decreasing because we saw that the frequency and wavelength are inversely related for electromagnetic waves. But that brings up the last question: what really happens to these electromagnetic waves when they go and hit something? For example, what happens to these radio waves when, you know, from the Wi-Fi, they go to our phones?

Well, our phones also have a tiny antenna inside of them. Now in the transmitter, an antenna, electrons vibrate, giving you electromagnetic waves. In the receiver antenna for a phone, the exact opposite happens: the electromagnetic waves will fall on it and make the electrons go up and down. And that's how the energy from this oscillating electron is transferred to the electrons over here. Pretty cool, huh?

But now we may be wondering, well, how do you transmit any information, though? For example, the radio waves that we would be using in Wi-Fi could be about 5 GHz, meaning 5 times 5 billion Hertz. That means the electrons over here are oscillating about 5 billion times per second. When the waves go and hit over here, the electrons inside the antenna of our phones will also oscillate 5 billion times per second. But where is the information being transferred over here?

How do you transform? How do you transfer information? That's a great question! In order to transfer information, we need to modulate this signal. What do you mean by that? Say, for example, this is a signal that we want to send from the transmitter to the receiver.

Let's imagine radio transmitters and radio receivers. You can imagine this is, I don’t know, a signal that represents some song for example. How do you transmit that? Well, what we do is we take our radio wave and we modulate it. An example of this would be you can change the amplitude of this radio wave according to the message signal, and it will look somewhat like this.

I know it sounds—it looks very complicated, but what's important is: look at the amplitude. The amplitude nicely matches the message signal. This is called modulation. In fact, this is called amplitude modulation because the amplitude of this radio wave is changing according to the message. This is the message that is sent by the radio transmitter.

When your radios receive it, the electrons—look, even though they're vibrating with the same frequency, the amplitude of the vibration will keep changing according to the message, and that's how your radio receivers will detect, you know, what the message was. This is what AM stands for, amplitude modulation.

There are other kinds of modulation as well. There's something called frequency modulation, phase modulation, and so on. The idea is you change some property of your radio wave in accordance to the message that you want to send. But anyways, since this is a continuous wave that is being modulated, we call this an analog modulation.

Your Wi-Fi does not do that. Your Wi-Fi does what we call digital modulation because it doesn't send analog signals like this; your Wi-Fi sends messages in bits of zeros and ones. This is a digital message; they are discrete. Unlike the analog signal, you only have two values; you either get zero or one, nothing in between.

So here we do digital modulation. What would that look like? Well again, if we were to modulate the signal using this message, we might get a modulated signal that would look somewhat like this. Again, notice the amplitude is either maximum or zero, okay, representing your zero or one bits that you get.

Again, when this goes and hits the receiver antenna in your phone, for example, based on the electrons will vibrate accordingly and will be able to detect what this message was. This is called digital modulation. All of this is an oversimplification, and there's so much more to it.

In a nutshell, this is how our communication works today—wireless, long-distance communication, satellite communication, what you want to talk about—all of that is by using, by harnessing the power of electromagnetic waves. It's just fascinating stuff!