As far as the energetics of light goes, for example in the photoelectric effect or Compton scattering, a "photon" is the correct "unit" amount. The energy associated with a particular photon depends on its wavelength. The relationship is E = h*c/lambda where
E = the energy of the photon, h is Planck's constant, c is the speed of light and lambda is the nominal wavelength of the light.
The "size" of a photon is a bit tricky to define. The problem has to do with the Heisenberg Uncertainty Principle. You cannot measure the "position" and "momentum" (or equivalently its length and energy) of a photon simultaneously. The product of the uncertainties is equal or greater than Planck's constant. So, the "length" of a photon depends on how accurately you want to define its wavelength.
LeeH
(published on 10/22/2007)
