Particle Nature of Light: The Photon

The photon as a particle of radiation. Einstein's equation (Section 11.7) works only if light genuinely behaves, in this kind of interaction, as a stream of localized, particle-like energy packets rather than a continuous, spread-out wave -- and such a packet is called a photon. A photon has several properties that mark it out as behaving like a particle:

• It carries a definite energy, $E=h\nu$, fixed entirely by the frequency of the light, and delivers this energy to whatever absorbs it in one indivisible unit, not gradually.
• It carries a definite momentum as well, $p=\dfrac{E}{c}=\dfrac{h\nu}{c}=\dfrac{h}{\lambda}$ (using $c=\nu\lambda$) -- a genuinely mechanical, particle-like momentum, even though a photon has no rest mass and always travels at exactly the speed of light $c$.
• In an interaction such as the photoelectric effect (or, in a closely related process, the scattering of a photon off a free electron, known as the Compton effect), energy and momentum are conserved between the photon and the electron exactly as they would be in an ordinary two-particle collision.
• A photon carries no electric charge, so it is unaffected by electric or magnetic fields, and it is not deflected by them (a genuine point of difference from a charged particle such as an electron).

Intensity, reinterpreted. In this particle picture, the intensity of a beam of light is reinterpreted as the number of photons crossing unit area per unit time, multiplied by the energy each individual photon carries -- rather than as a continuous energy density spread smoothly over the wavefront, as the classical wave picture had it. …