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Doppler Effect: Instructor's Guide

Main Ideas

  1. Using hyperbolic triangle trig to derive an important consequence of special relativity.

Students' Task

Estimated Time: 10 minutes, including wrap-up

Consider an inertial observer moving to the right in the laboratory frame who is carrying a flashlight that is pointing to the left. Suppose the moving observer turns on the flashlight just long enough to emit one complete wavelength of light. What wavelength is seen in the laboratory frame?

Prerequisite Knowledge

  • basic understanding of special relativity
  • basic understanding of hyperbolic triangle trig



  • Using student work if possible, show how to solve this problem using hyperbolic triangle trig.

See also the discussion in the text.

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