Wien's Displacement Law (OCR A Level Physics): Revision Note

Wien's Law

Black Body Radiator

• An ideal black body radiator is one that absorbs and emits all wavelengths. 

  • A black body is a theoretical object, however, stars are the best approximation there is

• The radiation emitted from a black body has a characteristic spectrum that is determined by the temperature alone

The intensity-wavelength graph shows how thermodynamic temperature links to the peak wavelength for four different bodies

Wien's Displacement Law

• Wien’s displacement law relates the observed wavelength of light from an object to its surface temperature, and can be summarised as:

The black body radiation curve for different temperatures peaks at a wavelength that is inversely proportional to the temperature

• This relation can be written as:

• Where:

  • λ_max = the maximum wavelength emitted by an object at the peak intensity (m) 

  • T = the surface temperature of an object (K) 

• This equation shows that the higher the temperature of a body:

  • the shorter the wavelength at the peak intensity, so hotter objects tend to be white or blue, and cooler objects tend to be red or yellow

  • the greater the intensity of the radiation at each wavelength

Table to compare surface temperature and star colour