Photon momentum. Light pressure. Technical uses of the photoeffect
A photon has zero mass but has energy E = hν and momentum: from E = pc, p = E/c = hν/c = h/λ. Photons falling on a surface give it momentum, so light exerts pressure. For light of intensity I (W/m²) falling perpendicular to the surface, the pressure on a fully absorbing surface is p = I/c and on a fully reflecting surface p = 2I/c, because on reflection the photon's momentum changes by 2p. This pressure is tiny (sunlight near the Earth presses on an absorbing surface with about 4.5 μPa, a few millionths of a pascal); P. N. Lebedev measured it in 1900 with a delicate torsion balance, and it is now used in designs of lightweight solar-sail spacecraft. Technical uses of the photoeffect: a photoresistor's resistance falls when it is lit (light sensors, automation); a p–n junction solar cell converts light energy directly into electrical energy (one cell gives an emf of about 0.5 V, so cells are connected in series); in a light-emitting diode current in a p–n junction produces light by recombination of electrons and holes, and the colour depends on the semiconductor.
Take a solar calculator or garden lamp and watch how it works with different lighting (shade, room, window). Do not look straight at the Sun and never point a laser pointer at eyes or people.