Electromagnetic waves

0:00- [Narrator] What's common between a wifi router, our bodies, and an incandescent bulb? We all give out electromagnetic waves. But why do we do that? And why are they, why are they all so different? And how do we use some of them for wireless communications? Let's answer all of them.

0:15Let'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.

0:29And that, as you can see in this animation, 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.

0:46But how do you produce these electromagnetic waves, you ask? Well, just accelerate charges. Accelerating charges will change electric fields. It'll 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.

1:03If you continuously wiggle an electron up and down, it'll continuously keep losing energy as electromagnetic waves. So accelerated charges create electromagnetic waves. So if you consider the wifi antenna, for example, you'll find that there'll be electrons oscillating up and down very quickly, okay?

1:20And 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 body. There are charges inside that are vibrating because just because of the temperature, they're just vibrating over there.

1:36And that will, again, generate electromagnetic waves, which happen to be infrared radiation. And 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.

1:48And 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 rays over here, light rays over here? What's the difference between them? In all the cases, it's the charges that are going up and down, right?

2:05Then why do we get different kinds of electromagnetic waves? What's the difference between them? To answer that question, we need to understand some of the key properties of these electromagnetic waves. So let's say this is our electromagnetic wave. Remember, it has both electric and magnetic fields.

2:18I'm not showing both of them. Imagine this is just electric field, and it's moving. So you can imagine the field is, 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?

2:29That's what I mean. So that technically is called the amplitude of the wave. So this height is what we call the amplitude. So 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, I'm not, there is nothing physical over here.

2:53These are electric and magnetic field values, right? So 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.

3:13So let's compare only these two points on the wave and let's see what happens to them as I'll just only compare to those two points and see what happens to them as the wave moves forward. I'm gonna move the wave to the right. So notice they will start oscillating.

3:26They'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. So you can think of phase as kind of representing where they are in their cycles of oscillation. They're both, these two are in phase because they reach their valleys together.

3:45They reach the zero together, they reach the peak together, and so on and so forth. So 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?

4:00Let's see. Let's see, again, let's just 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. And so no, they are not oscillating together. They're not in phase with each other.

4:15In fact, we say they're out of, 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.

4:28You 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.

4:50And it is usually represented by the symbol lambda, okay? So again, if you go back to our wave, and 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?

5:12Well take any point you want and then measure in one second how many waves go past that point. Number of waves passing through a point per second is what we call frequency. You can think of it as number of waves passing through a point per second, okay? So the unit of the wavelength is meters because you're measuring distance.

5:34And what is the unit of frequency? Well, it's number of waves per second. So the frequency would be numerator is 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.

5:58Now, 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.

6:13Let's say we have a wave which has a wavelength of two meters and it has a frequency of four hertz. How do we figure out the speed of this wave? Well, what we're gonna do is, let's imagine there's a wave that starts from here and we'll just wait for one second, okay?

6:26So wait for one second. Now in that one second, I know four waves that wanna pass by. So let me just draw them. So our wave are gonna start from here. It's a peak. So four waves are gonna pass by in one second. So here's wave number one. Peak to peak is one full wave, okay?

6:46Wave 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.

7:02And 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.

7:22Why don't you pause and try? All right, okay, so since the wavelength is two meters and I have four waves, the total distance traveled by the wave would be two times four. It would be eight meters. So in general, if the wavelength is lambda, this would be lambda.

7:47And if the frequency is F, there would be F number of waves over here. That means the total distance traveled would be F times lambda in one second, and therefore the speed of the wave would be F times 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 turns out things are a little bit more complicated.

8:12It depends on which wave we are dealing with. So now let's come back to electromagnetic waves. 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 three times 10 to the power eight meters per second.

8:35We often call this C, the speed of light, speed of light in 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.

8:55If 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?

9:15Well, because the charges over here are oscillating at different frequencies. In your wifi, 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.

9:29And so now, you get a slightly higher frequency waves, which you call infrared waves. Inside the light bulb, it's even higher frequency, which we get, call visible light because we can see it. And just to give you a feeling for numbers, okay? So your radio wave that's coming from your wifi is in the order of about a billion hertz, meaning the electrons over here are oscillating about billion times per second, okay?

9:52And when it comes to infrared, it becomes 10 to the 13. For visible light, it becomes 10 to the 14. You can even have higher frequency waves, which goes into ultraviolet rays. You have X-rays, you have gamma rays, and so on. And you can also see as the frequency increases, the wave length is decreasing because we saw that the frequency and wavelength are inversely related for electromagnetic waves.

10:14But 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 wifi it goes to our phones, for example? Well, our phones also have a tiny antenna inside of them.

10:30Now, in this transmitter antenna, electron vibrates giving you electromagnetic waves. In the receiver antenna for a phone, the exact opposite happens. The electromagnetic waves will fall on it and makes the electrons go up and down. And that's how the energy from this oscillating electron is transferred to the electrons over here.

10:48Pretty 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 wifi could be about five gigahertz, meaning five times, 5 billion hertz. That means the electrons over here are oscillating about 5 billion times per second, and then the waves go and hit over here.

11:08The electrons inside the antenna of our phones will also oscillate 5 billion times per second. But what, where is 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.

11:23What do you mean by that? Say for example, this is a signal that we want to send from the transmitter to 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?

11:36Well, 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.

11:57The amplitude nicely masses, sorry, nicely matches the message signal. This is called modulation. In fact, this is called the amplitude modulation because the amplitude of this radio wave is getting, is changing and according to the message. This is the message that is sent by the radio transmitter.

12:18And 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.

12:33This 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.

12:49But anyways, since this is a continuous wave that is being modulated, we call this an analog modulation. Your wifi does not do that. Your wifi does what we call digital modulation because it doesn't send analog signals like this. Your wifi sends messages in bits and zeros and months.

13:09This is a digital message. They're discreet. See, 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 modulate signal that would look somewhat like this.

13:30Again, notice the amplitude is either maximum or zero, okay? Representing your zero or one bits that you get. And again, when this goes and hits the receiver and receiver antenna in your phone, for example, based on the electrons, will vibrate accordingly and we'll be able to detect what this message was.

13:49This is called digital modulation, and 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, whatever you wanna talk about.

14:04All of that is by using, by harnessing the power of electromagnetic waves. It's just fascinating stuff.