The momentum of a photon of wavelength lambda is

  • A. h divided by lambda
  • B. h times lambda
  • C. zero, since a photon has no mass
  • D. mc

Explanation

A photon has zero rest mass but still carries momentum h over lambda, which is what allows radiation pressure and the recoil seen in the Compton effect. Solar sails are propelled by exactly this momentum transfer. Assuming that no rest mass means no momentum is the standard misconception here.

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