A photon also known as a corpuscle or a quantum is the basic unit of electromagnetic radiation.
You will remember that the electromagnetic spectrum is made up of different radiations. These radiations are waves and as such they are energy being moved from one place to another.
The photon is simply a small packet of energy and a beam of electromagnetic radiation is just a stream of these photons travelling in a particular direction.
So if the photon is a packet of energy then you should be able to determine the energy of a photon of electromagnetic radiation.
The following equation is used:
Energy of a photon = Planck’s constant * frequency of electromagnetic radiation
E = h * f
Planck’s constant = 6.63 x 10-34 m2 kg s-1