Module 5 : MODERN PHYSICS
Lecture 24 : PHOTONS
PHOTONS
Planck's explanation of black body radiation was revolutionary as it suggested that atoms could exchange energy only in multiples of quantum of energy. Five years later, in 1905, Einstein put forward a theory of photoelectric effect which suggested that the quantum of energy was not a property associated with the radiation emitted by atoms but is a property of radiation itself. Radiation, according to Einstein's theory consists of discrete bundles of energy, called photons. Thus, electromagnetic energy is seen as a collection of photons. A photon is characterized by an energy $ E$, related to the frequency by the relationship
 
$\displaystyle E = h\nu$
  Further, each photon carries a momentum $ p$ given by
 
$\displaystyle p = \frac{E}{c}=\frac{h\nu}{c}$
  In Einstein's special theory of relativity, the energy of a particle of rest mass $ m$ and momentum $ p$ is given by
 
$\displaystyle E^2 = p^2c^2+m^2c^4$
  which implies that photons have zero rest mass.
   
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