AQA A-Level Physics/The photon
A photon is an elementary particle; it is the force carrier for the electromagnetic force. A photon has no rest mass, this allows for interaction at long distances. A photon is what we called a packet, it is a burst of electromagnetic waves.
The photon theory was first established in 1905 by a man you've probably heard of, Albert Einstein. Einstein used his ideas to explain photoelectricity. This is the emission of electrons from a metal surface when light is directed at the surface. Einstein imagined photons as flying needles and he assumed that the energy, E of a photon depends on it's frequency. The equation for photon energy is as follows E=hf This is where h is a constant of which we refer to as Planck's constant. It has a value of 6.63x10^-34 Joule seconds