• summary_type: concept
• word_count: 170

A photon is a quantum of electromagnetic radiation carrying energy E = hf = hc/λ and momentum p = h/λ = E/c. It has zero rest mass but nonzero relativistic mass m = E/$c^{2}$ = hf/$c^{2}$. The most useful JEE conversion: E(eV) = 1240/λ(nm). A source of power P at wavelength λ emits n = Pλ/(hc) photons per second. Radiation pressure on an absorbing surface is P_rad = I/c; on a perfectly reflecting surface it doubles to 2I/c. The intensity of light I = nhf/A, where n is the photon flux per unit area. Stefan's law (P = σ$AT^{4}$) and Wien's law (λ_max T = $2.898 \times 10^{-3}$ m·K) describe black body radiation. The Compton effect (wavelength shift $\Delta$λ = (h/mc)(1-cosθ) upon X-ray scattering from electrons) provides direct evidence of photon momentum. Photons interact with matter in an all-or-nothing fashion — there is no partial absorption.