Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a 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 that 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. The forces generated by radiation pressure are generally too small to be noticed under everyday circumstances; however, they are important in some physical processes and technologies. This particularly includes objects in outer space, where it is usually the main force acting on objects besides gravity, and where the net effect of a tiny force may have a large cumulative effect over long periods of time. 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 km (9,300 mi). Radiation pressure from starlight is crucial in a number of astrophysical processes as well. The significance of radiation pressure increases rapidly at extremely high temperatures and can sometimes dwarf the usual gas pressure, for instance, in stellar interiors and thermonuclear weapons. Furthermore, large lasers operating in space have been suggested as a means of propelling sail craft in beam-powered propulsion. Radiation pressure forces are the bedrock of laser technology and the branches of science that rely heavily on lasers and other optical technologies. That includes, but is not limited to, biomicroscopy (where light is used to irradiate and observe microbes, cells, and molecules), quantum optics, and optomechanics (where light is used to probe and control objects like atoms, qubits and macroscopic quantum objects). Direct applications of the radiation pressure force in these fields are, for example, laser cooling (the subject of the 1997 Nobel Prize in Physics), quantum control of macroscopic objects and atoms (2012 Nobel Prize in Physics), interferometry (2017 Nobel Prize in Physics) and optical tweezers (2018 Nobel Prize in Physics). Radiation pressure can equally well be accounted for by considering the momentum of a classical electromagnetic field or in terms of the momenta of photons, particles of light. The interaction of electromagnetic waves or photons with matter may involve an exchange of momentum. Due to the law of conservation of momentum, any change in the total momentum of the waves or photons must involve an equal and opposite change in the momentum of the matter it interacted with (Newton's third law of motion), as is illustrated in the accompanying figure for the case of light being perfectly reflected by a surface. This transfer of momentum is the general explanation for what we term radiation pressure.
Discovery
Johannes Kepler put forward the concept of radiation pressure in 1619 to explain the observation that a tail of a comet always points away from the Sun. The assertion that light, as electromagnetic radiation, has the property of momentum and thus exerts a pressure upon any surface that is exposed to it was published by James Clerk Maxwell in 1862, and proven experimentally by Russian physicist Pyotr Lebedev in 1900 and by Ernest Fox Nichols and Gordon Ferrie Hull in 1901. The pressure is very small, but can be detected by allowing the radiation to fall upon a delicately poised vane of reflective metal in a Nichols radiometer (this should not be confused with the Crookes radiometer, whose characteristic motion is not caused by radiation pressure but by air flow caused by temperature differentials.)
Theory
Radiation pressure can be viewed as a consequence of the conservation of momentum given the momentum attributed to electromagnetic radiation. That momentum can be equally well calculated on the basis of electromagnetic theory or from the combined momenta of a stream of photons, giving identical results as is shown below.
Physical origin Electromagnetic radiation consists of an electric field E and a magnetic field B related by:
where n is the unit vector directed along the propagation direction of the radiation, and c is the speed of light in vacuum. The fields are transverse and satisfy the relation E ⊥ B ⊥ n {\displaystyle \mathbf {E} \perp \mathbf {B} \perp \mathbf {n} } . These properties are notably reflected in the Panofsky–Phillips equations.
When this electromagnetic field reaches an electric charge q initially at rest, the electric field exerts a force (F = qE) on it, setting it into motion. Once the charge is in motion, it is also subject to a magnetic force ( F = q ( v × B ) ) {\displaystyle (\mathbf {F} =q\left(\mathbf {v} \times \mathbf {B} \right))} exerted by the magnetic component of the radiation. Since the velocity v acquired by the charge is directed along E, and the magnetic force is given by F = q ( v × B ) {\displaystyle \mathbf {F} =q\left(\mathbf {v} \times \mathbf {B} \right)} , it follows that this force is directed along n. Moreover, one can note that the electric field exchanges energy with the charge, while the magnetic field imparts momentum to it.
The magnitude of the power P transferred by the electric field to the charge is given by:
which implies:
The magnitude of the force F transmitted by the magnetic field to the charge is, according to F = q ( v × B ) {\displaystyle \mathbf {F} =q\left(\mathbf {v} \times \mathbf {B} \right)} , given by:
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