# Q & A: Properties of photons

Most recent answer: 10/22/2007
Q:
Is "photon" the proper term for the smallest measurable amount of light? Or is there something smaller? How large is a photon?
- Eric Santana (age 38)
Hollywood, Florida
A:
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)