The biochemist George Wald (1906-1997) discovered in 1958 that vision is best explained in terms of photons. The molecule C₂₀H₂₈O is called retinal and is a component of both rods and cones in the retina. It can exist in two forms, one straight and one bent. When a photon strikes the molecule it changes its shape from bent to straight. The shape change creates an impulse in the nerves in the retina. The energy of the photon (and thus the wavelength of the light) that causes the transition depends on the other molecules, called opsins, in which the retinal molecule is embedded.

A wave carries energy continuously over time; the more energy in the wave, the faster the energy is transferred. A particle, on the other hand, delivers its energy all at once. When an atom either absorbs or emits light, the transfer is almost instantaneous. Therefore light interacts with an atom like a particle.

The idea that light comes in packets of energy was first stated by Albert Einstein (1879-1955) in 1905. He called the packet a light quantum. The quantum was given the name photon in 1926. The photon has no mass or charge, but it does carry angular momentum. It always moves at the speed of light, c.

Each photon carries an amount of energy E = hf, where fis its frequency. Therefore a photon of blue light has more energy than one of red light. The energy carried by a beam of light depends both on the frequency and the number of photons per second leaving the source.