# Wien's law and black body emition

#### Gabokappa

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?

Related Astronomy and Astrophysics News on Phys.org

#### James R

Homework Helper
Gold Member
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.

#### Gabokappa

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 im not sure about

#### Gabokappa

apparently it happens both ways, depends what wavelength your looking at say UV and infra red

#### James R

Homework Helper
Gold Member
Can you quote the relevant parts of your references, or link to them, or at least say what they are?

#### Nereid

Staff Emeritus
Gold Member
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?

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving