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Physics32. Electromagnetic WavesThe Electromagnetic Spectrum

So, one thing that we need to know is the speed of these EM waves. We said was equal to C. Right, and C is 3 times 10 to the 8 meters per second. But, there is a relationship between C and some of the other quantities that we're going to talk about, which is the following: C is equal to F times lambda. F is the frequency. Lambda is the wavelength. C is, of course, speed of light. Okay, so meters per second is F, which is 1 over seconds times lambda, which is meters. Okay, so that's a relationship that we're dealing with. And now, let's think about the full electromagnetic spectrum. So, we have a big long line here. And, let's label this one frequency F -- measured in Hertz. And, it's going to start down here at the low end at something like a kilohertz. And at the high end, it's going to go up to 10 to the 22 Hertz. And then going the other way, we have lambda. And, lambda is measured in meters. Those are inversely related and C is a constant, right? If F goes up, lambda has to go down. if F goes down, the lamda that has to go up. And, this over here is on the order of 10 to the minus 13. And, it goes all the way over to on the order of 10 to the 5. Okay? So, these things aren't going to line up perfectly well, but they're going to be roughly close. So down here at the low end of the spectrum, we have what is called the radiofrequency. Okay, and the radio frequency extends up to about 10 to the 9 in Hertz, or roughly 1 in meters. So, radiofrequencies contain, of course, power lines way down here at the bottom end, AM radio, FM radio, and television. Okay? These are all operating in the radiofrequency. The next region is called microwaves. And, microwaves are of course like your microwave oven. But also, your Wi-Fi signals. And, this is gonna go up to on the order of 10 to the 10, or 10 to the 11. Okay? So, this is what we talked about in class. People are a little concerned about cellular phones and Wi-Fi signals because it's really close to the same frequencies that you use for your microwave oven to heat your food. After that, we get infrared. Okay? Infrared goes up to about 10 to the 14 - 10 to the 13 - 10 to the 14 somewhere in there. And then, after infrared, we get to visible And visible goes from 10 to the 14 up to about 10 to the 15. If we keep going to higher frequencies, we get to UV. UV gets up to about 10 to the 16. And then, we get to x-rays. X-rays go all the way up to about 10 to the 19. And then, the very last bit is something called gamma rays. The gamma rays can go as high as 10 to the 22. So, infrared that's stuff like heat lamps. Visible is, of course, the Sun. UV would be black lights. X-rays you get when you go to the dentist. Gamma rays you only get from objects like quasars. Okay? And on the scale here, 10 to the 10 corresponds to about 0.1. And then, we get to 10 to the minus 6 up here, and 10 to the minus seven, and ten to the minus eight, and ten to the minus ten, and so on. Okay, these numbers aren't going to match up exactly and in fact, these divisions are not precise. Okay, they just say this region is microwaves but nobody says this is the hard cutoff for the microwaves on the right side and the hard cutoff on the left side. These are general regions except with regards to visible. Okay, visible we describe in exquisite detail and that's largely because we operate on the visible wavelengths. Remember we asked that question a second ago: How do you detect electromagnetic waves? Well, you can use your car antenna and detect radiofrequency waves. But if you want to detect visible light, how do you do it? How do you guys detect visible light? You're doing it right now, right? You use your eyeballs. Visible light is what you see. That is the light that you see. And, it's sort of interesting that there's this huge range available and it's all electromagnetic waves, all the way from gamma rays down to radiofrequency waves. It's all the same stuff it's all electromagnetic waves. But, we only detect visible which is a really small portion of the spectrum. If somehow you could detect UV, and infrared, and x-ray, then microwave, you could see a lot more of what's happening in the world. And, in fact, you're familiar with one area which is this right here, infrared. Because you've all seen the Arnold Schwarzenegger movies where they use infrared goggles. Okay? Which is adapted from the military. And, the idea is you can detect infrared waves. And if you have a special device, infrared goggles, you can turn it to visible. And so now, you can see things out there that are emitting infrared light, even though they're not emitting any visible light. And, this is kind of weird, right? Because if we turned off all the lights in this room and turn off the board, you guys would be in the dark And if I looked at you, I can't see you. But, if I could look at you with my infrared goggles, all of a sudden I would be able to see you. Because as humans sitting here, emitting light, it's in the infrared region. You're at a temperature of 98.6 degrees. You are emitting light that is about 10 microns long. And so with my goggles, I'd be able to see you. Maybe, I'll bring some in we'll try that sometime. It'll be good. Okay. Questions about this electromagnetic spectrum? Alright. There's one other point that I wanted to make. And this is something that we're not gonna see a lot of in this class. But, I wanted to give you a taste for where you might be going in your physics education, which is this idea of quantum physics. Has anybody ever heard of the wave-particle duality in quantum physics ? Okay, there's this notion in quantum that everything behaves like a wave and a particle simultaneously, even things like electrons. They're not really, just point particles they also have this wave-like property too. And, it turns out that when you're looking at this end of the electromagnetic spectrum, everything behaves like waves. Everything looks like a wave. It doesn't look like a particle. But up at this end of the spectrum, everything behaves like a particle. It doesn't behave like waves. So, gamma rays really act like little tiny bullets, little particles just flying through the universe. It's very hard to discern any wave-like properties at this end. And, it's very hard to discern any particle-like properties at this end. So, at either end you have waves over here and particles over here. But, where we sit in the middle in this visible spectrum light acts like a wave and a particle. And this is the basis for the wave particle duality. And light in this region has been shown to behave like both simultaneously. Okay, depending on how you design your experiment, you might look for the wave properties, different experiments, you might do the particle properties. But over here at gamma rays, particles. Over here at radio waves its waves. So, it's kind of interesting and we'll learn more about that when we get to a bit of quantum physics.