The energy of a photon is calculated in two ways as follows:
- If the frequency f of the photon is known, then we use the formula E = hf.This equation was first suggested by Max Planck and hence is referred to as Planck’s equation.
- Similarly, if the wavelength of the photon is known then the energy of the photon can be calculated using the formula E=hc/λ. In the equation, h is the Planck’s constant, c is the speed of light and λ the wavelength of the radiation.
The units of eV, or electron volts, are used as the units for photon energy, instead of joules. The value of h = 4.1357 × 10-15eV s, is used to provide more reasonable energy scale for photons.