Radiation pressure

Radiation pressure is the force per unit area exerted when electromagnetic radiation transfers momentum to matter. Its magnitude depends on the radiation’s intensity and on how the surface absorbs or reflects it.

Photon momentum: The momentum carried by a photon, proportional to its energy and inversely proportional to the speed of light. Radiation pressure arises when photons transfer this momentum to a surface.
Radiant intensity: Radiant power transmitted per unit area, measured perpendicular to the direction of propagation. For a directed beam, intensity sets the energy—and momentum—arriving at each area.
Solar sail: A spacecraft propulsion system that uses momentum transferred by sunlight to a large reflective sail. It turns the Sun’s radiation pressure into continuous propulsion without propellant.
James Clerk Maxwell: Scottish physicist whose electromagnetic theory predicted that light carries momentum and exerts pressure. Maxwell derived radiation pressure from electromagnetic field theory.
Electrostatic pressure: Mechanical stress produced by an electric field acting on charged matter or conducting surfaces. It can also push surfaces, but arises from electric fields rather than transported radiation.
Maxwell stress tensor: A tensor describing electromagnetic momentum flow and the stresses exerted by electromagnetic fields. It expresses radiation pressure as electromagnetic stress across a surface.
Radiant energy flux: The rate at which radiant energy crosses a unit area, including its direction. Flux clarifies how beam orientation changes momentum delivered to a surface.
Poynting–Robertson effect: The gradual loss of orbital energy by dust grains moving through sunlight because of radiation drag. Radiation pressure’s relativistic tangential component makes orbiting dust spiral inward.
Adolfo Bartoli: Italian physicist who derived radiation pressure thermodynamically in the nineteenth century. His argument established pressure from radiation before direct measurement.
Gas pressure: Force per unit area produced by particles colliding with a surface or exerting stress within a fluid. Both are pressures, but radiation pressure transfers momentum through electromagnetic fields instead of molecular collisions.

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