Radiation Pressure

Electromagnetic waves carry momentum p = E/c (per photon, p = h/λ). When they hit a surface, they exert a pressure called radiation pressure.

Perfect absorber: momentum E/c is fully transferred → pressure P_abs = I/c.

Perfect reflector: photon reverses direction, momentum change is 2E/c → pressure P_ref = 2I/c.

Partial absorber (reflectance R, 0 ≤ R ≤ 1): P = (1+R)·I/c.

Effect is tiny in everyday life — solar radiation gives ~5 μPa on absorbing surfaces and ~10 μPa on mirrors — but real and measurable.

Applications: solar sails (NEA Scout, IKAROS spacecraft) propel themselves without fuel; optical tweezers manipulate cells using laser radiation pressure; comet tails are pushed away from the Sun by both radiation pressure and solar wind.

Astrophysical importance: in massive stars, radiation pressure can dominate gravity (Eddington limit) — sets the maximum stable luminosity of a star.