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Â The way that electromagnetic radiation or light exchanges

Â energy with objects is mostly through the electric field.

Â So what happens is, the electric field of the light that's coming in acts on the,

Â a charged oscillator of the object that we're talking about,

Â and if the frequency, there's a Greek letter nu,

Â the frequency of the light is pretty much the same as the frequency of the,

Â this oscillator, then the energy from this light being carried

Â through the vacuum can be dumped into this oscillator and

Â you can heat the thing up through the vacuum.

Â So we're gonna talk about the light that comes off of an object,

Â because this is a two-way street.

Â If the light can come in and be absorbed by this oscillating piece of matter,

Â if you have this matter being oscillated just because it's warm,

Â it can also create light and send it back out.

Â And so we're gonna talk about the kinds of light that this object emits

Â by thinking about a spectrum, which is a plot of how bright the light is

Â as a function of the different frequencies or wavelengths or colors or

Â however you want to describe the different kinds of light.

Â So it's a plot that looks kind of like this.

Â It's got an intensity on the vertical axis.

Â And then we're going to use wave numbers on the horizontal axis

Â as our index of colors.

Â So the units on the intensity axis are watts per square meter per wave number.

Â And the reason why that's done is because that way if you have

Â a range of light color, say between one and two wave number units,

Â one and two waves per centimeter, for example, then the area

Â under this curve is going to be equal to watts per meter squared per n times n.

Â And the n's will cancel, leaving us just with a total of watts per square meter.

Â So that means that these plots are drawn so that the area under the total curve

Â is the total energy leaving the object in nice

Â units that you now understand of watts per square meter of the surface of the object.

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So what those spectra look like are these sort of humps.

Â I've drawn three of these different humps.

Â This one's sort of off the top of the chalkboard there.

Â Because you get different curves, depending how, on how warm the object is.

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So as the object gets warmer,

Â the peak energy, which is where the intensity is the strongest,

Â shifts in this direction toward a shorter wavelength or

Â higher wave number, more energetic light.

Â So you probably had known this before by thinking about sort of red

Â hot and white hot.

Â So an object that's just at room temperature is shining light, but

Â it's all in the infrared, so

Â we can't see it with our eyes that can only see in the visible range.

Â But as the object gets hotter and hotter, it starts to, the tail starts to encroach

Â in the visible range and you start to see some red color there.

Â And then if it gets really hot, it can fill up the whole visible range and

Â that's when something gets white hot.

Â So white hot is much hotter than red hot.

Â You already sort of knew that, right?

Â The other thing about this spectrum is that

Â it gets much bigger as you get hotter.

Â And it turns out that there's a formula describing this.

Â It's right here.

Â The total energy in watts per square meter is given by these three terms.

Â The first is epsilon, which I'll explain next.

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The second is the Stefan-Boltzmann constant,

Â which is just a constant number you can look up in a book.

Â It never changes.

Â And the third is the temperature in Kelvins raised to the fourth power.

Â So you raise it to the fourth power, that means if you double the temperature,

Â the energy flux goes up by a factor of two to the fourth which is 16.

Â So it's a very very powerful function of temperature.

Â So back to this term now, epsilon.

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So [SOUND] if an object is a blackbody, it's as though

Â it's a musical instrument that has all of the notes.

Â So, like a piano that has all the keys, if you just, you know,

Â hit it with a big hammer or something, make all of the keys vibrate at once,

Â you'll get this big wall of sound like this.

Â But if you have a piano that's missing a bunch of strings in the middle and

Â you do the same thing, you'll get some low notes and you'll get some high notes, but

Â there will be some missing stuff in the middle.

Â So this is what they, what a physicist would call a blackbody, because it makes

Â a smooth blackbody curve like this, and the epsilon value for this would be one.

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So you'd put in a number one here.

Â And there's no units to epsilon.

Â It either goes for zero, if an object had no of these oscillators and

Â couldn't make any infrared light at all, to one if it had all of the notes and

Â could make all the different frequencies.

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Â