Radiation Pressure Calculator

Electromagnetic radiation carries momentum as well as energy. When light is absorbed or reflected by a surface, its momentum changes and the surface experiences a force. Radiation pressure is that force divided by illuminated area. Although the pressure is usually tiny in everyday settings, it becomes important for intense lasers, precision optical systems, stellar environments, dust dynamics, and proposed solar-sail spacecraft that accumulate acceleration over long periods. This calculator treats the source as isotropic, so its power spreads uniformly over a sphere. Incident intensity equals source power divided by four pi times distance squared. A perfectly absorbing surface receives pressure equal to intensity divided by the speed of light. A perfectly reflecting surface reverses the incident momentum and receives twice the absorbing pressure. Intermediate reflectivity linearly interpolates between those ideal limits under this simplified model. Multiplying pressure by the entered illuminated area gives the normal force. Several assumptions matter. The surface is taken to face the radiation directly, all non-reflected energy is absorbed, the beam is uniform across the stated area, and the source radiates equally in every direction. A collimated laser beam does not follow inverse-square spreading in the same way; for that case, use measured beam intensity or power divided by actual beam area. Oblique incidence reduces the normal momentum transfer, while transmission, diffuse scattering, spectral behavior, and imperfect beam overlap require a more detailed optical model. The calculator uses the exact defined speed of light, 299,792,458 meters per second. Results in pascals and newtons may therefore appear in scientific notation. For solar applications, use solar irradiance at the desired distance rather than treating the Sun as a point source unless the equivalent luminosity model is appropriate. For engineering decisions, confirm optical properties at the actual wavelength and temperature, evaluate thermal loads, and include structural and pointing uncertainties. This result is an idealized physics estimate, not a substitute for optical, thermal, or spacecraft-system analysis.