Black bodies... explanation pls (1 Viewer)

[Giant Lobster]

What's a black body? And why did they [jackaranda] have to use a hole in an induction oven as an example?

I'd understand it much better if they used, say, a perfect sphere made of some kind of matter that absorbs all light falling onto it, and emits light at wavelengths characteristic to its temperature... but no, they used a stupid hole to confuse things up. so I may as well ditch my understanding of what a BB is...

so umm, yeah, what is it?

 

[Chand]

isnt it just a perfect emitter and absorber of energy...i think Planck further refined it by saying that it send and absorbed packets of energy (something similar to that) and thefore is sent in quantised amounts. Thats where einstein got his idea of photons from.

 

[underthesun]

they made our lives harder.. considerably.

 

[Giant Lobster]

exactly, I thought alone those lines too. If it were a perfect emmitter / absorber of light then why would the book use a thing with a hole in it and describe the light as enterring / exitting the hole?

I dont get it, why cant the light just simply be absorbed by a surface and not a hole?

geez, they probably tried to simplify an explanation and in the process added much confusion.

 

[PoLaRbEaR]

i was just reading through a passage which says that an oven with a small hole seems to be the perfect black body...

'The wavelength or frequency distribution of blackbody radiation was studied in the 1890s by Wilhelm Wien of Germany. It was his idea to use as a good approximation for the ideal blackbody an oven with a small hole. Any radiation that enters the small hole is scattered and reflected from the inner walls of the oven so often that nearly all incoming radiation is absorbed and the chance of some of it finding its way out of the hole again can be made exceedingly small. The radiation coming out of this hole is then very close to the equilibrium blackbody electromagnetic radiation corresponding to the oven temperature. Wien found that the radiative energy dW per wavelength interval d has a maximum at a certain wavelength m and that the maximum shifts to shorter wavelengths as the temperature T is increased.'

 

[Giant Lobster]

So the point of using a small hole is to control the probability of a beam of light from escaping once it has entered? is that it?
hmm I'm not quite sure on what you mean by the equilibrium of blackbody radiation. At equilibrium does it mean the amount of energy the oven is getting from the hole matches the amount it emits through that hole?
What about the external walls of the oven, doesnt that also affect temperature? like... if a beam missed and hit the wall, then the oven has just increased temperature in theory. So doesn't that also affect the wavelength peaks etc..
I still dont really quite understand the need for the hole. Cos if they put that oven in a really illuminated room, the external walls of the oven will contribute to most of the absorbing of energy rather than the hole.

 

[PoLaRbEaR]

I think the whole thing with this oven concept was just for measurement and to figure out the relationship between the radiation and frequency, or wavelength...so i guess maybe when they do the experiment they just shine a light through the hole and then measure the amount of energy coming bak out [and if they miss then they start again...thats just my guess so if anyone cares to correct me...]
Since the hole is small compared to the oven, the energy coming out of the hole is minute compared to the rest that's inside the oven and thus does not affect the distribution inside it...
The equilibrium means that the oven, upon reaching a certain temperature while being heated up, will emit radiation at the same rate as it is receiving it...

 

[Giant Lobster]

oh ok that clears alot up. thanks

but i still dont understand 100%...
why did they use an 'oven' and not just a box? the fact that they use an oven implies its on. is it?

 

[PoLaRbEaR]

I think an oven is the same as a black box...its not specifically refering to an oven as in the electrical appliance in the kitchen to heat up food...that's just called an oven because it's pretty much a black box...[anymore care to correct me?]..sounds logical to me...
Another black body is a cavity...and when they say cavity they dont mean the hole in ur tooth, but rather a hollow place..i guess that would sort of be the same as the oven concept...???