The electromagnetic spectrum

0:00- [Narrator] You probably know that you need light to see things, but what exactly is light? What are its features, and what makes them have so many different colors? And what really happens when light gets brighter? What's really going on over there? And what about those ultraviolet and infrared radiation we see, which we can't see with our own eyes, is that light, too?

0:22So many different questions. Let's get to the bottom of all these. Light can be described using a wave model. We've talked about this before. When you disturb water at a particular point, it makes those molecules vibrate, which in turn makes the molecules surrounding it vibrate, and so on and so forth.

0:38And that's how the disturbance propagates outward as a ripple. And if you have a continuous disturbance, like, you know, you have water dripping from a tap, for example, then you have a continuous wave or a periodic wave. And the same thing, looking from the side, looks somewhat like this.

0:53And we've talked about various features of waves, like the speed at which the wave travels; the number of waves passing through a point is what we call the frequency, and the distance between consecutive peaks or consecutive valleys, crest and troughs, we call it the wavelength.

1:06And the amplitude represents the maximum change from the equilibrium position. Now guess what? Light can be modeled exactly like this, meaning it has a wavelength, it has frequency, it has amplitude, and it has a particular speed. But wait, here's a question. In water waves, it's the water particles that are going up and down, that's vibrating, producing a wave, right?

1:27So the water acts as a medium. If you consider sound waves, the air acts as the medium. What about light? What is oscillating up and down? What is acting like a medium over here? Well, we do see light traveling through air and then it traveling through glass. So is that the medium for light?

1:46No, because light can also travel in vacuum. For example, this is how we get light from different stars. There is vast vacuum in between. There's absolutely no medium over there. So the big question is, then, what's vibrating when it comes to light? What's going up and down?

2:02To answer that question, let's look at a PhET simulation over here. I have a charge over here, and it's producing an electric field. An electric field is the reason why if you place another charge over here, it experiences a force. Similarly, magnets interact with each other using magnetic fields, okay?

2:17Now, here's a question. What's going to happen if I were to wiggle this charge? Well, let's see. I'm gonna wiggle it. Ooh, do you notice ripples going out? Just like the ripples in the pond. This ripple going out is ripples of electric field. And there'll also be magnetic field generated, which is not shown on here.

2:35But this ripples in electric and magnetic field is what we call electromagnetic wave. It's a wave because you can see that the disturbance is getting propagated. But what's wiggling over here? Of course, the charge is wiggling over here, but what's the medium?

2:52What's wiggling when it comes to the waves? It's not matter; it's the field. The charge wiggling makes the electric and magnetic fields wiggle. And that, you know, vibration, that disturbance, gets propagated outwards as an electromagnetic wave. And that electromagnetic wave is light.

3:11Okay? And so, just like any wave, look, it will have properties like the speed. So you can see this particular speed at which the wave is emanating outwards. In vacuum, the speed of an electromagnetic wave happens to be 300,000 km/s. It's very fast, the fastest thing in the universe.

3:31And it also has frequency. So, for example, you can see this is low frequency, less number of waves traveling per second. And then we can have high frequency, more number of waves traveling per second, because I'm wiggling it with higher frequency. And then, just like before, it can have amplitude.

3:47You can see amplitude represents the maximum change from the equilibrium position. So it has some amplitude now, and I'm gonna reduce the amplitude. Ooh, smaller amplitudes, smaller wiggles. Larger amplitudes, larger wiggles. Let's try to make even larger amplitudes.

4:01Oh, my God, big wiggles. So this is how we can model light, as an electromagnetic wave. So what's oscillating up and down in the wave is the electric and magnetic fields. So now let's try to answer our original questions. What decides the color of the light? Well, turns out it's the frequency.

4:20Again, going back to our PhET simulation, if we turn on light with low frequency, it tends to look red to us. Low-frequency light looks red to us. And I have a graph over here which shows the oscillations of the electric field, and the sound is gonna help us get a sense of the frequency.

4:35So here are the low-frequency sounds now. (frequency lightly pulsing) Right. Low frequency. Now let's see what happens if I turn up the frequency. Here it goes. (frequency quickly pulsing) Can you hear that? High frequency, right? It's oscillating much faster.

4:50High frequency. These are actually electromagnetic waves, okay? So electric and magnetic fields are oscillating over here, but high frequency tends to be bluish. Lower frequency tends to be reddish. And all the other colors will come in between. So it's the frequency that decides the color of the light.

5:05So again, what we register as red-colored light is basically light of lower frequency. And what we register as bluish-colored light are lights of higher frequency. That's what decides the color. Okay, what about the brightness? What decides the brightness? Well, let's see. Again, let me turn on the sound.

5:21The brightness is decided by the amplitude. Let me make the amplitude very tiny and see what happens. Here it goes. It's not so bright anymore. It becomes very dim. Look at it. Low amplitude means low brightness. On the other hand, if I were to increase the amplitude, let me just make it very, very high amplitude, (frequency quickly pulsing) The sound increases, just to let us know that the amplitude has increased.

5:43This is high amplitude. This is a brighter light. So the brightness comes from the amplitude. This means when the light is turning brighter, what you can imagine is the amplitude of the light is becoming bigger and bigger. That's amazing, isn't it? But here's something that's even more mind-boggling.

6:04So what we understand so far is that if you consider high-frequency light, or light that has lower wavelength, smaller wavelength, it tends to be bluish. And as the wavelength becomes larger, the frequency tends to become smaller, and the light tends to become redder and redder.

6:19But guess what? The visible light that we see is a very tiny portion of the entire electromagnetic spectrum. This visible light has a wavelength that is roughly of the size of a bacteria, and our eyes are sensitive to that. But you can have wavelengths much larger and much smaller than this.

6:40So let's explore them. What happens if the wavelength becomes even larger, or if the frequency becomes even smaller than red? Well, when the wavelength reaches the size of, you know, the thickness of a human hair, for example, we call that the infrared. Now, we can't see it with our own eyes, but guess what?

6:57Our bodies tend to send out infrared radiation all the time. And so there are devices that can detect this infrared radiation and then convert that into visible light so we can see it. We call them thermal cameras, and they can be used for night vision. Okay, what happens if the wavelength becomes even larger or if the frequency becomes even smaller?

7:15Well, when the wavelength reaches the size of, say, a baseball, we now call it microwaves. And they have the ability to heat up water, and that's why microwave ovens exist. And that's how they can heat up food, because they heat up the water in your food. Okay, but what if it goes even larger?

7:31Well, now, if the wavelength reaches the size of buildings, for example, we call them radio waves. They can be used for long-distance communications. Okay, let's go on the other end of the spectrum. What happens if the wavelength becomes even smaller than that of the visible light, meaning even higher frequency?

7:46Well, if the wavelength becomes small enough, as small as, say, viruses, for example, we call them ultraviolet waves. And again, this is invisible to us, but turns out certain bees can see these waves. The Sun also produces a lot of UV rays, and they can cause damage to our DNA.

8:02And that's the reason why we use sunscreens. Okay, but what if the wave becomes even smaller? When they reach the size of, for example, molecules, we call them x-rays. One of their cool properties is they can penetrate skin, but they get absorbed by your bones, and that's why they can be used to take images of your bones.

8:18Okay, what if we go even smaller? Well, when the wavelength reaches the size of the atomic nucleus, we call them gamma radiations. Gamma rays have the shortest wavelengths, or the highest frequency, in the entire electromagnetic spectrum. And they're produced in nuclear reactions, like radioactive decays, for example.

8:37And so, there you have the entire electromagnetic spectrum.