My friend asked me the other day, “What is radiation? Light is radiation, it’s electromagnetic radiation man!” – but then what’s with all the radioactive materials that give off so-called “radiation” of Beta and Alpha particles, huh?
Well, in general, he’s right, and in general I’m wrong (except when in those cases, when I’m right!). The radiation people generally refer to is Electromagnetic Radiation. It’s what warms us when we step into the sunlight, it’s what burns us when we’re near a bonfire or similarly, it’s what burns us when we choose leather seats rather than upholstery (leather seats, die!).
Everything is always radiating – mostly. If you were to point at something wherever you’re at right now, I could tell you, “yeah, it’s probably radiating, dude.” Even at room temperatures, you’re giving off electromagnetic radiation too, a.k.a. we’re all glowing. Only at invisible wavelengths though. The total amount of radiation you give off increases a lot with an increase in temperature.
If you took a metal bar and heated it up, it starts glowing, don’t it? This is because, the total radiation it gives off is increasing, and as it does, the portion of that that’s in the visible wavelengths of light increases as well. Here, look at this graph, drop some knowledge in this b****!
(FYI, I just Googled “wavelength-intensity creative commons” so there shouldn’t be any problems, right? Right.)
This graph puts a few physics facts on display. It shows the power radiated from an idealized object called a Black Body at various wavelengths, more on that later. But we can immediately see something occurring here: At all temperatures, the curve is rather similar in that it always has a peak somewhere. Another thing to note, it seems that as the temperature increases, this peak shifts to the left. This is the wavelength of maximum intensity, and it’s position is described rather simply by Wien’s Displacement Law:
Where b is Wien’s displacement constant. This has been experimentally measured to be around b=2.8977729(17)×10−3 m⋅K.
Ever notice how, even when the hot metal is dull (i.e. not glowing like the Eye of Sauron) you can still feel it’s heat from a distance? That’s the infrared light rays tickling your heat-sensitive bits. They’re light rays that are invisible to us mere mortals (though there are plenty of mortal snakes that somehow know this trick). Wonderful isn’t it?
The expression that a guy name Stefan and another guy named Boltzmann kinda collaborated to derive for the total power output of radiation of a heated body is:
Where A is the surface area of the object, is the Stefan-Boltzmann constant, T is the absolute temperature (measured in Kelvins), and is the emissivity of the object. The emissivity ranges from 0 to 1, and in so doing, represents how “efficient” the object is at hurling your radiation back at you. A black body is that which has an emissivity of 1.
There are some things in this universe that a pretty damn close to being perfect black bodies – astronomical objects like stars, for instance, very nearly match the wavelength-intensity graph of a black body at the same temperature. Take a look.