A photon is a particle of light, a packet of electromagnetic radiation. The energy of the photon depends on its frequency (how fast the electric field and magnetic field wiggle). The higher the frequency, the more energy the photon has. Of course, a beam of light has many, many photons. This means that really intense red light (lots of photons, with slightly lower energy) can carry more power to a given area than less intense blue light (fewer photons with higher energy).
The speed of light (c) in a vacuum is constant. This means more energetic (high frequency) photons like X-rays and gamma rays travel at exactly the same speed as lower energy (low frequency) photons, like those in the infrared. As the frequency of a photon goes up, the wavelength () goes down, and as the frequency goes down, the wavelength increases. The equation that relates these three quantities for photons: .
Because wavelength and frequency are determined by each other, the equation for the energy contained in a photon can be written in two different ways:
One of the strangest discoveries of quantum mechanics is that light and other small particles, like photons, are either waves or particles depending on the experiment that measures them. When light passes through a prism they spread out according to wavelength.
Bombard metal with light, and it displays a particle side of its nature, where only photons that have more than a specific amount of energy release electrons.
This experiment, called the photoelectric effect, is what won Einstein his Nobel Prize. Photons with insufficient energy can hit metal, yet won't knock any electrons loose. Photons that exceed a threshold energy usually do knock the electrons loose, however as the photon's energy becomes much greater than necessary the likelyhood that it does eject an electron diminishes. Thus a low total energy beam of violet light might eject electrons from a particular metal, where a high energy red beam fails to eject one. Since each photon in the red beam is lower energy, there are many more of them. This effect was part of what led to the quantum revolution in physics. Classical physics, and intuition, both wrongly conclude that the total energy of the beam would be the most important factor in ejecting electrons.
This phenomenon is important for the physics of the photovoltaic cells.