Radiation Pressure by Reflection (using particle model: photons)


Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. Radiation pressure implies an interaction between electromagnetic radiation and bodies of various types, including clouds of particles or gases. The interactions can be absorption, reflection, or some of both (the common case). Bodies also emit radiation and thereby experience a resulting pressure.

The forces generated by radiation pressure are generally too small to be detected under everyday circumstances; however, they do play a crucial role in some settings, such as astronomy and astrodynamics. For example, had the effects of the sun’s radiation pressure on the spacecraft of the Viking program been ignored, the spacecraft would have missed Mars orbit by about 15,000 kilometers.

This article addresses the macroscopic aspects of radiation pressure. Detailed quantum mechanical aspects of interactions are addressed in specialized articles on the subject. The details of how photons of various wavelengths interact with atoms can be explored through links in the See also section.

Electromagnetic radiation is quantized in particles called photons, the particle aspect of its wave–particle duality. Photons are best explained by quantum mechanics. Although photons are zero-rest mass particles, they have the properties of energy and momentum, thus exhibit the property of mass as they travel at light speed.

he generation of radiation pressure results from the momentum property of photons, specifically, changing the momentum when incident radiation strikes a surface. The surface exerts a force on the photons in changing their momentum by Newton’s Second Law. A reactive force is applied to the body by Newton’s Third Law.

The orientation of a reflector determines the component of momentum normal to its surface, and also affects the frontal area of the surface facing the energy source. Each factor contributes a cosine function, reducing the pressure on the surface. The pressure experienced by a perfectly reflecting planar surface is then as shown here.

Related formulas


PreflectPhoton pressure (Pa)
EfEnergy flux (watt/m2)
cSpeed of light
aAngle between the surface normal and the incident radiation (degree)