Hello. I'd like to talk to you about black body radiation. Black body radiation is the light that is given off by any object that has a temperature above absolute zero. And that's just about everything. And so any object emits black body radiation. And it emits that black body radiation in a special way with a certain spectrum, with a certain distribution of its energy spread over the wavelengths of the light. And you'll see what I mean by that in just a second. So what I have here is a lamp. You can see the light bulb right here and inside the light bulb there is a straight filament. That's what heats up. As I run an electric current through the lamp, the light bulb will get hotter and hotter and hotter and it will glow brighter and brighter and brighter. And it's going to be giving off black body radiation. And what we're going to do is look at the spectrum of that radiation. Look at the colors that light has within it. So to do that, we're going to use this. This is a diffraction grating. All it is is a piece of plastic and it can break up light into its components covers. The same way drops of water can break up light to form the continuous spectrum of a rainbow. We're going to break this black body radiation up into its continuous spectrum. And to do that, I'm just going to take this diffraction grating and I'm going to put it over the lens of the camera. And so from here on, you'll be looking through the diffraction grating and I'll be turning on this lamp. So here I come. Let's take this diffraction grating and I'm going to put it right on the lens of the camera just like that. So right now, you're looking through the diffraction grating and when I turn the lamp on, we'll see the spectrum of black body radiation. Now to see it better, I'm going to turn the room lights off. And so, let me turn on the lamp for a second and I'll turn the room lamp off so you can see the black body radiation spectrum a bit better. So here we go. I'm turning on this light. I'm going to turn it on fairly bright at the beginning. There we go. And I think you can see that there to the right of the lamp is the spectrum, the rainbow spectrum of the black body radiation. So I'm going to go turn off the lights in the room and I'll be right back. The room lights are off now and you can see the spectrum of black body radiation. But this lamp is getting off. You can see it's a continuous spectrum and that is all the colors of the rainbow are in that spectrum. The colors blend smoothly from one to the other. Over on the left side, we have the shortest wavelength, which is the violet light and over on the right hand side, we have the longest wavelength, which is the red light and all the colors in between. Now the wavelength of light is actually fairly short. On the blue side, the wavelength of violet light is about 400 nanometers. A nanometer is about 1 billionth of a meter. On the red side, we have a longer wavelength. That's about 700 nanometers and that's 700 billionths of a meter. Now what's interesting about black body radiation is the way the spectrum changes when the temperature of the emitting object changes. So I'm going to change the temperature of this lamp just by turning it down. So let's see what happens as I turn this lamp down a bit. Watch out with the spectrum changes. Here we go. Turning it down. Down. Down. Down. And take a look. First of all, you might notice that the lamp itself looks a lot redder. The lamp is much cooler and the lamp is giving off a lot more red light than it did before. So red light here is dominant at low temperatures. Now I'm really going to increase the temperature of the filament by running a higher electric current through it. The filament's going to heat up. Watch how the spectrum changes. It's increasing. Notice the blue is coming in. And as I continue to turn this up, it's not the blue color. It's the blue color. That our brightness no longer dominated by red. And so for a very hot filament, it gives off mostly light that is at the blue end of the spectrum. Whereas for a cool filament, most of the light is emitted in the red end of the spectrum. And that's an agreement with Vien's law. Vien's law says that cooler objects emit more red light and hotter objects emit more blue light. And that explains the colors of the stars. A hot star is going to emit more blue light. And so it's going to look bluish white to our eyes. Whereas a star we surface is cool is going to emit more red light. And in that case, the star will look reddish to our eyes. So cool stars emit red light, more red light than other colors. Because hot stars emit more blue light than other colors. And that explains the colors of the stars. For example, I'm going to put a picture of the constellation of Orion on the screen. And if you take a look at the star beehive cheese, you'll notice that that appears reddish. Beetle juice is in fact a red giant star. And the red giant star surface is very cool. And so Beetle juice looks reddish. On the other hand, if you look at Rijl, the star Rijl appears white hot. And in fact, it is that bluish white Rijl tells you the bluish white color tells you that Rijl is a very hot star. And so a strong verse finding black body radiation very useful because it tells them about the temperatures of the star's surface. I hope you've enjoyed this demonstration of black body radiation. And I will talk to you later. And I will fade one last time turning the lamp down. And then all the way off. Bye-bye.