Wien's law and black body emition

In summary, it is relatively straight-forward to work out what the observed spectrum of a blackbody - beyond the Earth's atmosphere - would be like, taking account of the absorption of the atmosphere.
  • #1
Gabokappa
3
0
Hi i was just wondering you when black bodies emit all spectrum of electromagnetic radiation. Now are atmosphere absorbs a particular part, therefore the peak wavelength and temperature appear to be different according to the rules of wiens law. Now does Earth atmosphere absorb Shorter wavelengths so that the peak shifts to the larger wavelength there fore black body say like a su appear cooler, or does the atmosphere absorb the longer waves, there fore peak shifts to the shorter waves and seems hotter?
 
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  • #2
The peak doesn't shift.

If some frequencies are absorbed, they just aren't present in the spectrum any more (or their intensities are lower), and so the spectrum is not perfect black-body any more.
 
  • #3
James R said:
The peak doesn't shift.

If some frequencies are absorbed, they just aren't present in the spectrum any more (or their intensities are lower), and so the spectrum is not perfect black-body any more.

No the peak does shift i read it in various places. Its slightly different then absorption lines. The peak wavelenght in the wiens law equation shifts. And its due to this shift that stars when observed from Earth have a slightly different temperature. But it depends which wavelengths the atmosphere absorbs. Thats what I am not sure about
 
  • #4
apparently it happens both ways, depends what wavelength your looking at say UV and infra red
 
  • #5
Can you quote the relevant parts of your references, or link to them, or at least say what they are?
 
  • #6
A belated welcome to Physics Forums, Gabokappa!

The Earth's atmosphere is opaque to UV (we could make a more precise statement, e.g. plot the absorption as a function of wavelength), and more or less transparent into the IR, except for various bands due to absorption by water, etc (again, we could make this a more precise statement).

It is relatively straight-forward to work out what the observed spectrum of a blackbody - beyond the Earth's atmosphere - would be like, i.e. taking account of the absorption of the atmosphere. We could even plot this as a function of the temperature of the blackbody.

Astronomers have been aware of these factors for decades, if not centuries, and have developed quite sophisticated techniques for removing the effects of atmospheric absorption from the data they take from the instruments attached to their telescopes.

Do you have a particular aspect of this topic you are interested in, Gabokappa?
 

1. What is Wien's law?

Wien's law, also known as the displacement law, states that the wavelength of maximum intensity emitted by a black body is inversely proportional to its absolute temperature. This means that as the temperature of a black body increases, the peak of its emission spectrum shifts towards shorter wavelengths.

2. What is a black body?

A black body is an idealized object that absorbs all radiation that falls on it and emits radiation at all wavelengths. It is a theoretical concept used to understand the behavior of thermal radiation, and real-world objects can be approximated as black bodies if they have high emissivity and low reflectivity.

3. Can Wien's law be applied to non-black bodies?

Yes, Wien's law can be applied to non-black bodies, but the results will not be as accurate. Real-world objects do not behave as perfect black bodies, so their emission spectra will not perfectly follow Wien's law. However, for objects that are close to being black bodies, such as stars, Wien's law can still provide a good approximation.

4. How is Wien's law related to the Stefan-Boltzmann law?

Wien's law and the Stefan-Boltzmann law are both laws that describe the behavior of thermal radiation. While Wien's law describes the peak wavelength of emission from a black body, the Stefan-Boltzmann law relates the total amount of radiation emitted by a black body to its temperature. The two laws are often used together to understand the properties of black bodies.

5. What practical applications does Wien's law have?

Wien's law has many practical applications, including in astronomy, where it is used to determine the surface temperatures of stars based on their emission spectra. It is also used in materials science and engineering to understand the behavior of thermal radiation in different materials and to design more efficient heating and cooling systems. Additionally, Wien's law has applications in fields such as chemistry and biology, where it can be used to study the thermal properties of molecules and biological systems.

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