Photon energy

Photon energy is the amount of energy a single photon carries. A photon is a tiny particle of light. Its energy changes with its frequency. The higher the frequency, the more energy the photon has. Photons with longer wavelengths, like radio waves, have less energy.

We can measure photon energy in different units. One common unit is the electronvolt (eV), and another is the joule. For very high-energy photons, such as gamma rays, we sometimes use bigger units like microjoules. These photons have much more energy than the ones we see with our eyes or in radio waves.

Formulas

Physics

A photon’s energy depends on how fast it vibrates, called its frequency. The higher the frequency, the more energy the photon has. We can calculate this energy using a simple formula: E = h f, where E is the energy, h is a constant called Planck’s constant, and f is the frequency.

We can also talk about a photon’s energy using its wavelength, which is how long the wave is. The shorter the wavelength, the more energy the photon has. The formula looks like this: E = h c / λ, where E is energy, h is Planck’s constant, c is the speed of light, and λ is the wavelength.

Electronvolt

Scientists often measure photon energy in a unit called an electronvolt, or eV. One eV is a very small amount of energy, equal to 1.602176634×10⁻¹⁹ J. To find the energy in eV from the wavelength in micrometres, we use this formula: E (eV) = 1.2398 / λ (μm).

For example, light with a wavelength of 1 micrometre, which is near infrared light, has about 1.2398 eV of energy.

Examples

An FM radio station sends out signals at 100 MHz. These signals create tiny energy packets called photons. They have very little energy — about 4.1357×10⁻⁷ eV. This is a very small amount, much smaller than the weight of an electron.

Very-high-energy gamma rays carry a lot of energy, from 100 GeV to over 1 PeV. This is 10¹¹ to 10¹⁵ electronvolts. During photosynthesis in plants, special molecules called chlorophyll catch red-light photons. These photons have just enough energy to help make a sugar called glucose from water and carbon dioxide.