Thermal Radiation & 'Black Radiation': Explained

[samreen]

what is 'black radiation' ? or specifically, the meaning of the phrase "radiation in equilibrium with matter"...in the context of thermal radiation. clueless here :'(

 

[mgb_phys]

I think you mean blackbody radiation.
A blackbody emits radiation with a certain characteristic that depends only on it's temperature - the sun is a good example.

An example of radiation equilibrium would be a planet around a star. The planet is cooler than the star and so absorbs radiation from it, as it heats up it then emits radiation to the colder space. It eventually reaches an equilibrium temperature where the amount of radiation emitted equals the amount recieved.

 

[Naty1]

mgb has it I think...Leonard Susskind uses the chapter title BLACK LIGHT (meaning radiation) in his book THE BLACK HOLE WAR...
I just happened to reread it yesterday and never really thought about the chapter title...he uses the term I think to cover black hole evaporation via Hawking radiation ( black body radiation) and discusses the black body nature of black holes...but Susskind sticks to blackbody radiation when discussing actual emissions.

There are some interesting illustrations here:
http://en.wikipedia.org/wiki/Black_body

 

[samreen]

nope. i mean black radiation. I am talking general here. we know from kirchhoffs work that any enclosure that is opaque to all kinds of radiation, will when maintained at any constant temperature, behave like an ideal black body, and emit radiation characteristic of that temperature, right? I've followed it so far.
what i don't follow is when the texts say that any speck of matter placed inside such an enclosure will attain equilibrium with the radiation filling the enclosure and the blacker it is, the faster this will happen. when it has, this speck starts emitting black radiation

 

[mgb_phys]

nope. i mean black radiation. I am talking general here. we know from kirchhoffs work that any enclosure that is opaque to all kinds of radiation, will when maintained at any constant temperature, behave like an ideal black body, and emit radiation characteristic of that temperature, right?

Correct, the inside of an oven is a pretty good black body
What confuses a lot of people is that glowing hot things, like stars or ligh bulb filamanets are also 'black' bodies.

what i don't follow is when the texts say that any speck of matter placed inside such an enclosure will attain equilibrium with the radiation filling the enclosure and the blacker it is, the faster this will happen. when it has, this speck starts emitting black radiation

image a piece of black material (eg soot covered metal) placed inside an oven, infrared radiation from the oven will hit the material, be absorbed and heat it up.
It is also emitting it's own photons, but as it's cooler there are fewer of these and they are a less energetic wavelength.
This will happen until the material reaches the same temperature as the oven - then it will emit radiation at exactly the same wavelength as those from the oven (the wavelength depends only on temperature) it will also emit exactly the same power/m^2 as the walls the oven - this is the equlibrium.

A highly reflective object put in the oven will reflect most of the heat radiation and so absorb very little and heat up much more slowly, this is why you put highly reflective insulation around things that need to work at very high or very low temperatures

 

[samreen]

it all seems so nice and obvious the way u put it. that was a ton of help :) thanx!