Definition of intensity in radiation

[TT0]

Hi, I recently read that as temperature increases, atoms move faster and have higher frequencies, shorter wavelengths and more intense radiation. What does intense mean? Does it mean in a substance there are a greater percentage of atoms emitting at higher frequencies or does it mean another thing?

Thanks

 

[drvrm]

What does intense mean? Does it mean in a substance there are a greater percentage of atoms emitting at higher frequencies or does it mean another thing?

Its not a straight relationship of greater percentage of atoms emitting radiation of higher frequency-- as when we see the intensity wavelength graph we find that wavelength of max. intensity times the temperature is a constant-and it shifts to higher frequency side when temp. is raised- it means new levels of the atomic states -higher in energy is getting involved in emission process more in number than the highest frequency region as the curve goes to a minimum.

 

[Anand Sivaram]

What drvrm mentioned is the Wien's law, but the black body emission power per unit area increases as T⁴ according to Stefan Boltzmann's law.

 

[TT0]

I still don't get intensity. What does it mean when a blackbody at a higher temperature emits a certain wavelength at a higher wavelength when compared to a blackbody at a lower temperature? Is intensity similar to watts but for wavelengths? So like how many wavelength goes through this area in a second?

 

[Anand Sivaram]

In a simple manner, consider this. Take a small metal strip and keep it on a gas stove. (Dont do it actually :) ), it will start getting heated up.
First you will see it as red hot, then becomes orange and then to yellow. The same you might have observed in an incandescent bulb with a dimmer circuit.

Normal gas stove can not support..but if you could heat it up even further, the object would become more and more whiter, means becoming more and more bluish white. For example, electric arc welding. where it emits very bluish white light...

Basically black body radiates every wavelength, but one of the wavelength is emitted maximum for a given temperature, that is called the peak wavelength. If you start heating objects the peak wavelength goes from Infrared to Red towards Blue, that is the peak wavelength starts reducing..Since ν = c/λ the peak frequency starts increasing.
The intensity is the total energy radiated in all wavelengths. That is proportional to T⁴.

In short, if you double the temperature, the peak wavelength would become half of the original, peak frequency would become double and the total energy radiated in all wavelength would be 16 times that of the original.

 

[TT0]

I see, I think I kind of get it now. Just to check when you said:

Basically black body radiates every wavelength, but one of the wavelength is emitted maximum for a given temperature

Do you mean that is the wavelength is emitted the most in comparison to other wavelengths emitted by the black body?

Thank you

 

[Anand Sivaram]

https://en.wikipedia.org/wiki/Black-body_radiation
If you look at the first diagram on Wikipedia you could see black body radiation curve against wavelengths, also the Y axis is the energy/wavelength, that is spectral radiance, or intensitiy per wavelength.

The wavelength corresponding to the peak of the curve is called the peak wavelength.

https://260h.pbworks.com/w/page/69071363/Beginnings of Quantum Mechanics
This link has a much better image of this.

Basically if you heat something to 7000K the peak is at 414nm (violet), Sun's peak is at 501nm (green) for a temperature of 5777K, for a halogen lamp of 3000K it is at 966nm (Infrared) etc. That does not mean that the Sun only emits 501nm, it emits all wavelength, but the peak is at 501nm.

 

[TT0]

I see, I get it now. Thank you very much! :)