radiation pressure when a photon is incident at an angle to the surface
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Answer:
Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength which is absorbed, reflected, or otherwise emitted (e.g. black-body radiation) by matter on any scale (from macroscopic objects to dust particles to gas molecules).The associated force is called the radiation pressure force, or sometimes just the force of light.
When light is reflected on a mirror, for example, the mirror is subject to some radiation pressure – i.e., to a small mechanical force which is related to the transfer of momentum from the radiation to the solid material. (Strictly speaking, the pressure is the force per unit area; it is related to the optical intensity rather than to the optical power.) Such forces have been first considered by the astronomer Johannes Kepler. James Clerk Maxwell, the creator of the famous Maxwell equations, has later substantially developed further the understanding of light forces; quantitative predictions have been derived from Maxwell equations. Light forces have later been experimentally proven by the Russian physicist Pyotr Lebedev in 1900
Applications of Radiation Pressure
Even though radiation pressure is usually very weak effect, there are some real or at least proposed applications:
Light forces on atomic particles are exploited in various methods of laser cooling.
Solar sails have been developed which could be used for the propulsion of small spacecrafts. Even though the obtained forces are quite small, they can substantially modify the momentum of an object if they are active during very long travel times. A crucial factor for practical applications is that such solar sails are made of very lightweight and yet durable materials.
Beam-powered propulsion of spacecraft may be realized with high-power lasers. However, such propulsion would provide only weak forces while requiring enormous amounts of power. On the other hand, the achieved momentum per kilogram of used fuel (e.g. a nuclear fuel) could be far higher than for rocket propulsion, for example. Therefore, that approach has been suggested for extremely distant travel.
It is possible to use the radiation pressure effect for measuring the optical power of a high-power laser beam . Although rather low mechanical forces have to be measured, an advantage is that the optical power to be measured does not need to be absorbed, so that it stays available for an application, and no substantial heat power needs to be dissipated.
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