Effective Mass Of A Photon — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Effective Mass of a Photon Used to describe the inertial properties of electromagnetic radiation (photons) when interacting with matter (e.g., radiation pressure), deriving momentum, or in Compton effect calculations. Begin with Einstein's Mass-Energy Equivalence relation: E = mc². Recall the energy of a photon from Planck's Quantum Theory: E = hν = hc/λ. Equate the two energy expressions: mc² = hc/λ. Rearrange to solve for mass (m): m = h/(cλ). The rest mass of a photon is always zero; this 'mass' is purely relativistic. Valid only in vacuum or where the speed of light is c. The concept applies to dynamic interactions, not static measurements. Students often mistake effective mass for rest mass, assuming photons have weight. Confusing the relativistic mass formula with classical kinetic energy relations (like K = 1/2 mv²), which do not apply to photons.