Additional Info

• ID Code: I2-04
• Purpose: Shows that higher temperature blackbodies radiate with shorter wavelengths
• Description:
  

  A variable transformer, or Variac, is connected to two identical incandescent light bulbs in parallel. These bulbs are viewed behind red and blue filters respectively. As the voltage is increased by the variac, the lights glow more brightly, and more light is seen through the blue filter relative to that of the red filter. Very little or no blue is seen at low voltages, whereas red is seen to be emitted even at very low voltages.

  Engagement Suggestion:

  • Ask students to compare this to other phenomena that emit light. Where else do you se this change of color with temperature?
  

  Background:

  Wilhelm Wien postulated in the 1890s that the power curve of blackbody radiation from an object could be computed from its temperature. His original calculations, obviously, did not take quantization into account; in modern practice, the revised calculations are still commonly referred to as Wien's Law.

  Note that this apparatus only works with incandescent lightbulbs. Fluorescent and LED bulbs do not produce their primary light through thermal excitation, and thus don't produce the same kind of blackbody spectrum.

• Availability: Available
• Loc codes: I2, PS1