Analyse the experimental evidence gathered about black body radiation, including Wien’s Law related to Planck’s contribution to a changed model of light

  • Black Body: A hypothetical object that absorbs all incoming radiation.
    • A practical example of an item that resembles a black body is a small hole in a box with a black interior.
  • When a black body becomes warmer than its surroundings, it becomes a perfect emitter.
  • Black Body Radiation: Electromagnetic radiation that is emitted by a black body.
    • The radiation emitted from a black body extends over all wavelengths of the electromagnetic spectrum.
  • Classical theory predicted that the intensity of radiation would increase indefinitely as wavelengths
  • However, experimental results showed that intensity peaked at a wavelength characteristic of the temperature of the black body.
  • Moreover, it also showed the wavelength of peak intensity of radiation emitted by a black body decreases as the temperature of the body increases.
    • This wavelength of peak intensity peak intensity only depends on the temperature of the body.
    • This relation is defined as Wien’s Law : Wiens law, where b = 2.9 x 10-3 Km

 

Plank’s proposal of a new model

  • The contradiction between classical theory and experimental results was resolved by Max Planck.
  • Planck proposed that radiant energy may be treated statistically as if it were exchanged in multiples of a certain discrete amount, or quanta.
  • Each frequency of radiation has a characteristic quantum of energy.
  • This is given by the equation: E = hf
    • where:
    • E = energy in joules (J)
    • h = a constant, now known as Planck’s constant (6.63 x 10-34 J s)
    • f = frequency in hertz (Hz)
  • This assumption allowed Plank to reproduce the results obtained experimentally from theory.
 
 

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)