A photon is a particle of light which essentially is a packet of electromagnetic radiation. The energy of the photon depends on its frequency (how fast the electric field and magnetic field wiggle, this needs better wording, for 'fast electric field' and 'wiggle'). The higher the frequency, the more energy the photon has. Of course, a beam of light has 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 ([math]\lambda[/math]) goes down, and as the frequency goes down, the wavelength increases. The equation that relates these three quantities for photons is: [math]c = \lambda f[/math].

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:

[math]E = hf[/math] or [math]E = \frac{hc}{ \lambda}[/math]

  • [math]E[/math] = energy of the photon
  • [math]h[/math] = the Planck's constant (6.62606957(29)×10-34 J·s )
  • [math]f[/math] = photon frequency
  • [math]\lambda[/math] = photon wavelength
  • [math]c[/math] = speed of light

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.

Contrarily, when bombarding metal with light, photons display the particle side of their nature. When photons exceed a specific energy threshold, they release energy to the metal's electrons. The excited electrons, having too much energy to maintain orbit, are ejected from the metal.

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 likelihood that it ejects 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 has lower energy, there are many more of them. This discovery is 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 photovoltaic cells.

To learn more about photons please visit hyperphysics photons and hyperphysics the quanta of light.