Photon energy

Photon energy is the amount of energy that a single photon carries. A photon is a tiny particle of light, and its energy changes depending on its frequency. The higher the frequency of the photon, the more energy it has. This also means that photons with longer wavelengths, like those in radio waves, have less energy.

We can measure photon energy using different units. One common unit is the electronvolt (eV), and another is the joule. Because photons can have a huge range of energies, sometimes we use bigger units like microjoules, especially for very high-energy photons such as gamma rays. These high-energy photons have much more energy than the photons we see with our eyes or those used 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 that sends out signals at 100 MHz creates tiny energy packets, called photons, with very little energy — about 4.1357×10⁻⁷ eV. This is a super small amount, much smaller than the weight of an electron.

Very-high-energy gamma rays carry a lot more energy, from 100 GeV to over 1 PeV, which 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.