Strålningsdruck

Strålningsdruck, or radiation pressure, is the pressure exerted by electromagnetic radiation on matter as photons transfer momentum to the surface they strike. Photons carry momentum p = E/c, so absorbing light imparts momentum F/c per unit area equal to the incident energy flux F; if the surface reflects light, the momentum transfer is doubled, giving 2F/c.

For oblique incidence, general expressions depend on the angular distribution of the radiation. For a perfectly absorbing surface with incident intensity I at angle θ to the normal, the pressure is P = (1/c) ∫ I cos^2 θ dΩ. For a perfectly reflecting surface, P = (2/c) ∫ I cos^2 θ dΩ.

In a cavity or isotropic radiation field, the pressure relates to the energy density u by P = u/3; for blackbody radiation, P = a T^4 / 3, where a is the radiation constant.

Strålningsdruck has several practical and theoretical roles. It was first measured experimentally by Pyotr Lebedev around 1900, confirming predictions from Maxwell’s theory. In space, solar radiation pressure can propel spacecraft equipped with solar sails. In astrophysics, radiation pressure contributes to hydrostatic balance in stars and to the dynamics of dusty environments around luminous stars; the Eddington limit describes the luminosity at which radiation pressure balances gravity. Tiny particles in orbit around stars experience Poynting–Robertson drag, a slow inward spiral due to the interaction of radiation with moving grains.

Measuring radiation pressure informs material properties such as reflectivity and absorptivity and remains a fundamental factor in the design of precision optical devices and models of stellar and interstellar processes.