[jgillespie]

How would you describe a blackbody?

I understand it to be a) mostly theoretical b) what people like Planck used to arrive at the particle nature of light and c) really hard to visualize.

Any help you could lend would be great.

John

[Grundalizer]

I don't find it that hard to visualize, in fact, I'm sure you've seen one before.

Yes, the idea of the blackbody being a "perfect" absorber of all EM radiation is obviously the theoretically part, but the important piece of information to know is that as the temperature of the black body rises, it may start to EMIT electromagnetic radiation as well, and that can be measured.

Have you ever turned on an electric stove?  The coils are black to begin with...and if you put your hand over them they are cool.  As their temperature starts to increase (while you keep your hand over them!!)  you will begin to feel warmth and heat even though they aren't red yet.  That's infrared radiation coming off.  As the coils get hotter and hotter, they get even start to emit electromagnetic radiation in the VISUAL spectrum, ie, they glow red/orange. 

Now, if you used the blackbody radiation equation, you should be able to equate the color you see the coil at to the temperature the coil is at.

What's basically happening is the electrons inside the coil are being excited to higher wavelengths, just like in light bulbs etc, and falling back down to ground state, emitting photons of differing wavelngths.  If you KEPT heating the coils up, they would eventually burn "white hot" which is even hotter than "red hot" because now you are moving towards the blue end of the spectrum as well, and including all wavelengths in the visible spectrum.

Another example of a pseudo black body is coals of a fire.  Watch how they are basically black balls of carbon, but when hot enough glow red and orange and seem to "shimmer" as their temperature changes ever so slightly in place to place...pretty cool to watch.