lists the different types that are generally recognised. Electromagnetic radiation is a wave, so has a wavelength λ and a frequency ν. It is also composed of particles called photons, and each photon has an energy E. These three quantities are related, and any of them may be used to define the type of radiation. The properties of electromagnetic radiation were first investigated by James Clerk Maxwell in 1864. He realised that in a vacuum they would travel at the speed of light, and he correctly deduced that light is radiation of this form.