A photon is characterised by either a wavelength, denoted by λ or equivalently an energy, denoted by E. There is an inverse relationship between the energy of a photon (E) and the wavelength of the light (λ) given by the equation:
where h is Planck's constant and c is the speed of light. The value of these and other commonly used constants is given in the constants page.
The above inverse relationship means that light consisting of high energy photons (such as "blue" light) has a short wavelength. Light consisting of low energy photons (such as "red" light) has a long wavelength.
When dealing with "particles" such as photons or electrons, a commonly used unit of energy is the electron-volt (eV) rather than the joule (J). An electron volt is the energy required to raise an electron through 1 volt, thus 1 eV = 1.602 x 10-19 J.
By expressing the equation for photon energy in terms of eV and µm we arrive at a commonly used expression which relates the energy and wavelength of a photon, as shown in the following equation:
The exact value of 1 × 106(hc/q) is 1.2398 but the approximation 1.24 is sufficient for most purposes.
To find the energy of a photon at a particular wavelength, click on the map above. You can also use the calculator below to enter the wavelength directly and find the energy of a photon anywhere on the electromagnetic spectrum.