Learn more physics!
why light is produced only when it is emmited by some source
- adnan (age 17)
Light has some conserved quantities: energy, momentum, angular momentum. Conserved quantities don't change over time. One can make light (e.g. photon pairs) for which the total momentum and angular momentum are zero. However, the energy of each part of the light is positive. So that had to come from something that already had energy. We call whatever that is the source of the light.
(published on 11/20/10)
Follow-up on this answer.