HSC Physics Blackbody Radiation

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The term black body radiation sometimes causes confusion in examination answers. Below is a tutorial sheet on this topic.

  1. Define the term blackbody. Is the "blackbody" the object, a cavity in the object or a small hole in the wall of the object?
  2. Define the term radiation as used in this context.
  3. Is the Sun a perfect blackbody? Is a lump of hot coal a perfect blackbody? Is the filament of a light globe a perfect blackbody? Is a hot oven a perfect blackbody?
  4. Sketch a graph showing the energy released per unit wavelength on the Y axis and wavelength on the X axis for a blackbody at a constant temperature. Is this graph the same shape as the "normal curve"? Is this graph the same shape as the distribution of particle speeds curve in an ideal gas?
  5. Sketch a graph showing the number of oscillators (vibrating atoms in the walls of a hot object) versus the energy of each oscillator at a constant temperature. This is a key difference between the quantum theory of light and the wave theory of light. The classical wave theory of light assumes that every oscillator has the same kinetic energy at a given temperature and this (incorrect) assumption leads to the ultraviolet catastrophe.
  6. Sketch a graph showing the energy released per unit frequency versus frequency for a blackbody at a constant temperature.
  7. Sketch a graph showing the peak wavelength in the black body spectrum versus the temperature of the object.
  8. What does the equation E=hf mean?
  9. Quantum theory predicts that an oscillator can only have certain particular energies. These are given by E=(n+1/2)hf where n=0,1,2,3.... Why is this result different to E=hf?
  10. *A perfect blackbody has a temperature of 5778 K. What fraction of the total energy released is contained between the wavelengths of 380 nm and 700 nm? This is the range of visible wavelengths coming from a blackbody at the same temperature as the photosphere of the Sun. [39%]
  11. In the previous question the temperature is doubled. Is the percentage of radiation emitted as visible light increased, decreased or the same? [decreased, 31%]