In effect, all Einstein did was to apply the equation E=hv to the electromagnetic radiation, instead of to the little oscillators inside the atom. He said that light is not a continuous wave, as scientists had thought for a hundred years, but instead comes in definite packets, or quanta. All the light of a particular frequency v, which means of a particular color, comes in packets that have the same energy E. Every time one of these light quanta hits an electron, it gives it the same amount of energy and therefore the same velocity. More intense light simply means that there are more light quanta (now we call them photons) that all have the same energy, but changing the color of the light changes its frequency, and so changes the amount of energy that is carried by each photon.

In some respects a photon acts as a particle, for instance when registered by the light sensitive device in a camera. In other respects, a photon acts like a wave, as when passing through the optics in a camera. According to the so-called wave-particle duality in quantum physics, it is natural for the photon to display either aspect of its nature, according to the circumstances