blackbody concept help (1 Viewer)

[VJ30]

can someone explain me all aboutblackbody radiation and quantum physics etc

1- what is a blackbody???
2- what does it mean that the energy radiated from a blackbody is quantised? (dont just tell me that it has discrete values and is not continuous- i already know that! but i dont get it...)

HELP PLEASE

thnks very much!

 

[Demise]

A blackbody is an object that absorbs and emits energy perfectly.

That aforementioned object can only absorb specific wavelengths (or frequencies) and can only emit radiation from such wavelengths. Theoretical data said that as frequency increases/wavelength decreases, the radiation emitted should increase exponentially and therefore would be able to reach infinite energy radiation, although this is impossible as it defies the laws of physics. Instead, experimental data showed that radiation emitted was capped at a specific wavelength and decreased exponentially passed that point. Also, that's the only way to explain the concept of the quantised radiation - it is not emitted continuously but is instead sent as packets. That's why in the photoelectric effect you can either release the electron or not, it doesn't just "heat" up - radiation is sent in specific quantas not as a stream of energy.

 

[VJ30]

^ well if I shine a light beam on a mirror surface, it will be completely reflected off, so can we call mirror a blackbody ???

 

[Demise]

^ well if I shine a light beam on a mirror surface, it will be completely reflected off, so can we call mirror a blackbody ???

No, a perfect mirror only reflects a light beam, it does not absorb it.

 

[nightweaver066]

1. A black body is a hypothetical object that absorbs all incident EM radiation.

2. First, lets delve a bit in to what the classical wave-theory of light predicted. It predicted that the intensity of black body radiation (BBR) as wavelength got smaller would approach infinity (known as the UV catastrophe). However this was inconsistent with experimental results which showed that the intensity dropped to 0 at wavelengths around UV.

Now Planck resolved this by stating that the energy absorbed and released by black bodies are quantised. This meant that light was composed of packets of energy proportional to the frequency of it (E = hf). Planck hypothesised that for certain quantum changes to occur, certain packets of energy must be absorbed or released. I say "certain" because packets of higher amounts of energy must be absorbed for larger quantum changes, and smaller amounts of energy must be absorbed for smaller quantum changes in an "all-or-nothing" fashion (half or a quarter of a packet of energy cannot be absorbed).

After a change to a higher quantum state, it changes back to a lower quantum state, releasing a packet of energy.

Thus, we can see that the energy radiated by a black body is quantised.

 

[GoldyOrNugget]

It predicted that the intensity of black body radiation (BBR) as wavelength got smaller would approach infinity. However this was inconsistent with experimental results which showed that the intensity dropped to 0 at wavelengths around UV, known as the UV catastrophe.

The UV catastrophe wasn't the discrepancy between the prediction and experimental evidence, it was the prediction itself. The idea that intensity would approach infinity as wavelength decreases to 0 was clearly impossible, and yet, contemporary mathematical models based on classical physics predicted this.

 

[nightweaver066]

The UV catastrophe wasn't the discrepancy between the prediction and experimental evidence, it was the prediction itself. The idea that intensity would approach infinity as wavelength decreases to 0 was clearly impossible, and yet, contemporary mathematical models based on classical physics predicted this.

Thanks, will fix it up

 

[VJ30]

thanks guys!