• Tags: photon, energy, momentum
• Difficulty: Foundation

A photon is a quantum of electromagnetic radiation with energy E = hf = hc/λ. Key properties: zero rest mass (photons always travel at speed c), momentum p = h/λ = E/c, and relativistic mass m = E/$c^{2}$ = hf/$c^{2}$. Photons carry no charge. The number of photons emitted per second by a source of power P at wavelength λ is n = P/E = Pλ/(hc). For JEE calculations, the conversion E(eV) = 1240/λ(nm) is extremely useful. Example: visible light (400-700 nm) has photon energies 1.77-3.1 eV. The intensity of light I = nhf/A, where n is the number of photons per unit area per unit time. When photons interact with matter, they are either completely absorbed or not at all — there is no partial absorption.