Radiation Pressure
Track photon momentum at absorbing, reflecting and transmitting surfaces and relate it to a measurable force.
Free theory articleTrack photon momentum at absorbing, reflecting and transmitting surfaces and relate it to a measurable force.
Concept & interpretation
Electromagnetic waves carry linear momentum as well as energy. Under normal incidence with irradiance I on a flat surface of area A characterized by absorbance a, reflectance r, and transmittance t (where a + r + t = 1), absorbed photons transfer their momentum p = h/λ while reflected photons reverse direction and transfer 2h/λ. The resulting radiation pressure is P = I(a + 2r)/c and the net mechanical force is F = PA.
Experiment
Explore how surface optical properties, irradiance, area, and wavelength affect photon momentum transfer in the Radiation Pressure module:
- Compare Surface set to Absorber versus Mirror at I = 300 W/m2 and A = 2 cm2. Notice that pressure and force double for the mirror (from 0.200138457119 nN to 0.400276914238 nN) while absorbed power drops to 0 W.
- Change Wavelength nm between 400, 600, and 800 at fixed irradiance. Observe that macroscopic pressure and force remain invariant because the change in per-photon momentum is exactly compensated by the inverse change in photon arrival rate.
- Select Transparent to verify that transmitted photons continuing in the same direction exert zero radiation pressure and zero force.