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If a black body at a temperature T radiates the maximum energy at a wavelength \(\lambda_{m}\) then to radiate the maximum energy at the wavelength \(\frac{\lambda_{m}}{3}\),its temperature should be

  • A.increased to 3T
  • B.increased to 9T
  • C.decreased to 3T
  • D.decreased to 9T
  • E.increased to \(\sqrt{3}\)T

Answer: A

The temperature should be increased to 3T.

By Wien's displacement law, \(\lambda_{m}T\) is constant, so the peak wavelength is inversely proportional to the temperature:

\[ T' = T\cdot\frac{\lambda_{m}}{\lambda_{m}/3} = 3T \]

A shorter peak wavelength means a hotter body, so 'decreased' is wrong. 9T and \(\sqrt{3}\)T apply power laws that Wien's law does not contain.

For example, a star with peak emission at 1500 nm would need to be three times hotter for its peak to shift to 500 nm.

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