lightpressure

Light pressure, commonly referred to as radiation pressure, is the force exerted on matter by electromagnetic radiation as photons transfer momentum upon absorption or reflection. Although individual photons carry tiny momentum, a beam of light can exert measurable force on objects with sufficient area or over extended exposure.

For a surface at normal incidence with reflectivity R (0 to 1) and light intensity I (power per unit area), the time-averaged pressure is P = I(1+R)/c, where c is the speed of light. For oblique incidence, the pressure is reduced by the square of the cosine of the incidence angle, giving P = I(1+R) cos^2 theta / c. Absorbing surfaces (R = 0) experience P = I/c; perfectly reflecting surfaces (R = 1) experience P = 2I/c. The units are pascals when I is expressed in watts per square meter.

History and measurement: Radiation pressure was predicted by Maxwell’s equations and first measured in the early 20th century, providing experimental confirmation of electromagnetic theory. The phenomenon is exploited in optical tweezers and related instruments, which use focused light to trap or manipulate microscopic particles. In astrophysics and space science, radiation pressure influences the dynamics of dust and gas around stars and contributes to the propulsion of solar sails or laser-driven spacecraft.

Applications and implications: Lightpressure enables propulsion schemes that rely on large, highly reflective surfaces and precise attitude control. It also necessitates accounting for micro-scale forces in high-precision satellites, detectors, and optical components, where even tiny pressures can accumulate over time.