section_path: Physical properties/Relativistic energy and momentum
section_name: Relativistic energy and momentum
paragraph_text: Relativistic energy and momentum
  
  In empty space, the photon moves at c (the speed of light) and its energy and momentum are related by E = pc, where p is the magnitude of the momentum vector p. This derives from the following relativistic relation, with m = 0:
  
  The energy and momentum of a photon depend only on its frequency (ν {\displaystyle \nu }) or inversely, its wavelength (λ):
  
  where k is the wave vector, where
  
  Since p {\displaystyle {\boldsymbol {p}}} points in the direction of the photon's propagation, the magnitude of its momentum is