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Emission of electromagnetic radiation

There are many different processes and phenomena that emit electromagnetic radiation. Humans have taken advantage of many of these processes to develop technologies that use electromagnetic radiation. Created by Sal Khan.

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  • blobby green style avatar for user Eleanor Q
    A few questions ...

    1. If matter at room temperature normally only seems to emit IR rays, then how cold would matter have to be to emit EM waves in the radio wave and microwave frequencies?

    2. How hot would matter have to be to emit EM waves in the X-ray and gamma ray frequencies?

    3. Since the sun emits EM waves in all frequencies, does this mean it has some hot parts to emit the higher energy waves, and some colder parts to emit the lower energy and frequency waves? Or is it in fact that all matter at a certain temperature will emit EM radiation of the highest energy and frequency possible for that temperature, PLUS all of the EM waves with a lower frequency than that maximum frequency?

    4. Since all EM waves carry at least some energy, do all of them increase the thermal energy of matter when they are absorbed by it and their energy is taken away by the matter?

    5. When an object absorbs visible light, it gains a certain amount of energy. It then emits IR radiation, which has a lower frequency and therefore less energy than visible light. So how has energy been conserved here? Does the object maybe emit more total photons of IR radiation than it absorbed photons of visible light, since each photon of IR radiation carries less joules of energy?

    6. Why does the trend happen to be that absorbed visible light is emitted as IR radiation anyway? Why, for instance, is absorbed visible light not just emitted as visible light again?

    Looking back, I realise this was more than a few questions, but I'm really curious about this stuff. Thanks so much!
    (5 votes)
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    • male robot hal style avatar for user Eamon
      Answer to Question 6:
      Due to law of conservation of energy, energy cannot be gained. Therefore, in cases where light is absorbed, much of the time, energy has gone to other purposes and has thus been converted, and therefore the re emitted light will be of lower energy. However, note that much of the time, we are not actually absorbing visible light, rather we are reflecting it. The visible light that bounces off a human and the thermal energy that can be captured in the infrared spectrum are completely separate.
      (2 votes)
  • eggleston green style avatar for user Ants Work Diligently
    So I have been researching absorption and emission spectra, and it seems to be that Emission spectra are all the (distinct) wavelengths that an object can(or will?) emit, while the Absorption spectra is all the wavelengths that it gets energy from, that it can absorb. Please verify or clarify, thank you.
    (1 vote)
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    • aqualine ultimate style avatar for user Inspiron13
      Emission spectra occur when an atom "emits" a photon when one of its electrons decays back to one of the lower orbitals, whereas an absorption spectra shows at what wavelength the light has been absorbed by the atom (ie: the electron used the photon to get to a higher energy orbital). This is why in an emission spectra, you only see the wavelengths of light that are emitted (the rest is black), whereas in an absorption spectra, you really do not see that specific wavelength because it has been absorbed by the atom, which corresponds to the black lines.
      (3 votes)
  • leaf blue style avatar for user RyanColorado2007
    At the beginning of the video, in the captions it says that someone called Norm is speaking, but the voice is clearly Sal.
    (2 votes)
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  • blobby green style avatar for user pandeysa
    What do you call electromagnetic waves with a frequency above the gamma wave frequency range?
    (1 vote)
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    • male robot hal style avatar for user Eamon
      There's not really a name for them, they are just rays with more energy (also the term gamma ray is not limited). It's not really important as the names are semi-arbitrary anyways. However also note that it is hard to gather enough energy to produce a ray of such extreme energy.
      (2 votes)

Video transcript

- [Norm] Let me ask you a seemingly simple question. I have a picture of fire here, and my question is, what is fire? Well, what would you say if I were to tell you that fire, as we see it, these flickering flames, it is nothing but hot air? And I know what you might be thinking. Hot air? "Norm, I've seen air that's hot, or I experienced air that's hot and I don't oftentimes even see the air, but here I clearly see something bright, something that's emitting light, something that's emitting electromagnetic radiation." And then what I would say to you, if you were thinking that, is it actually turns out that anything in our universe that has a temperature above absolute zero, zero Kelvin, which is pretty much anything that you will ever come across in your life, emits electromagnetic radiation. Objects with temperature aren't the only way to create electromagnetic radiation, but it is a major way that's happening all around us. Even if you were in a pitch-black room, you would be emitting electromagnetic radiation. A tree outside, even if it was dark outside, is emitting electromagnetic radiation. You might say, "Wait, but I don't see the tree," and that's because your eyes can only detect certain frequencies of electromagnetic radiation. If we look at this diagram right over here, we can see how we've categorized many of the frequencies, and you can see that up here, this is the frequency, this is the wavelength, and these are in powers of 10. So you can really view this as a logarithmic scale. And just over here, you can see that our eyes can only visibly see a small section of this logarithmic scale of frequencies. One of the things I like to wonder, if humans didn't have eyes, if we weren't able to detect even the small segment of the electromagnetic spectrum, would we even know that something like electromagnetic waves existed? But we can see you have gamma rays, x-rays, UV rays, infrared rays, microwave, FM, AM radio waves, long radio waves. In most hot air, the frequency isn't high enough for us to see it. So most hot air is going to be in the infrared range. Only if it gets hot enough will we start to see it, and that's what's happening with this fire here. And if you look closely at a fire, you might actually see that the location where the combustion reaction is happening, that that actually might be dark. And then right above that, you'll see some blue flame, and then you'll see, maybe if you look closely, some green or yellow flame, and then you will see the orange flame, and then you will see the red flame. And the reason why it might be dark right where the combustion reaction is happening is that might be very high energy electromagnetic waves. That would be in the UV spectrum. That would be at a higher frequency than what's visible, so to our eyes, it looks dark. And then as it cools, it goes through the visible spectrum. And then if it cools enough, it goes to infrared. But we human beings have built the capability to see beyond what our regular eyes can see. For example, these are what are often known as thermal images, but they're really just detecting the infrared range. So this is a picture of two dogs. It could be pitch-black outside. I mean, it could be the middle of the night, but because they have temperature, they are releasing electromagnetic waves, which we can detect. And this over here has a scale of what the temperature must be. So you can see the eyes of the dog are the hottest part right over here. You can also see thermal imaging of not only a hand, but after a hand has touched a wall. With our eyes, if you were to touch a wall for say 30 seconds, it doesn't look like the wall has changed at all, but if you were to look at the infrared, you would see that you would have heated up parts of the wall and you would be able to see the shape of the hand. And so you can imagine, we human beings, because of our ability to detect electromagnetic waves and explore electromagnetic waves, we've been able to leverage them more and more in our everyday lives. Thermal imaging itself has a lot of applications. Firefighters use it to find people, or to find flames in the middle of a lot of smoke. We have things like x-rays, which are high energy electromagnetic waves, which we can use to see through soft tissues. So we can see bones. This is an old image and it looks like they're using the x-rays kind of carelessly. You don't wanna be throwing that radiation around, but even today. I got an x-ray of my teeth just the other day when I went to the dentist. When you talk on your cell phone, the way that your cell phone is able to communicate is leveraging electromagnetic waves. This is another thing that's mind blowing to me, is that my little cell phone can actually emit electromagnetic waves in the radio part of the spectrum far enough to be received by a cell tower that could be 10, 20, and in certain cases, 30 or 40 miles away. Microwave ovens literally released microwaves, which are absorbed by our food, which heats up the food. And so I'll leave you there. The big picture here is that electromagnetic waves are all around us. It's most obvious to us in the visible spectrum, because that's what we can see. But there is a large continuum of different frequencies that the visible is only a part of. And we human beings have leveraged this phenomenon in all sorts of interesting ways, and I would suspect that we're just at the beginning of this exploration. Maybe you will come up with a new application of electromagnetic waves.