Radiation pressure or light pressure is the pressure that acts by absorbed, emitted or reflected electromagnetic radiation to a surface.

For absorption and emission of radiation pressure is equal to the intensity of the wave or of the irradiance in each case divided by the speed of light:

The units

With total reflection of the radiation pressure is twice as large.

## History and evidence

James Clerk Maxwell ran from 1873 from Maxwell's equations in the framework of electrodynamics that electromagnetic waves can exert a pressure on the body. He already showed that the radiation pressure perpendicular incident electromagnetic waves is equal to the volumetric energy density of the incident wave:

1876 ​​forwards Adolfo Bartoli from the existence of the radiation pressure from thermodynamical considerations. He argued that could be transmitted at a moving mirror due to the Doppler effect heat from a cold to a hot body by the reflection of light. To avoid the injury of the second law of thermodynamics, it is necessary that the light exerts a pressure on the mirror. The radiation pressure was therefore formerly named after its discoverers also Maxwell bartolischer pressure.

The first experimental confirmations came from Pyotr Nikolaevich Lebedev (1901) and Ernest Fox Nichols and Gordon Ferrie Hull ( 1903). The physicist Arthur Ashkin 1972 irradiated small plastic spheres with laser light and was able to observe a change in motion under the microscope.

## Explanation

The electromagnetic radiation can be considered both as a stream of photons, as well as an electromagnetic wave. From both models, the radiation pressure can be derived.

### Particle model

A photon of frequency carries the power

With Planck 's constant. This energy is due to the equivalence of mass and energy, equivalent to a mass of

Which moves at the speed of light. As a result, has a photon with the wavelength of the pulse with the amount:

The direction of the pulse is the moving direction of the photon. The total momentum is conserved in absorption, emission and reflection, that is, the interacting surface undergoes a change in momentum in the corresponding direction.

A plurality of photons, i.e., photon flux of the particle, causing a pulse at the absorption change per time unit, so a force of

This force acts at the incident angle to the surface normal of a surface element, to generate the pressure of

A reflected photon takes a pulse of the same amount with again, so that in the case of reflection results in twice the momentum transfer to the interacting surface and thus the double radiation pressure.

### Wave model

An explanation of the radiation pressure on the basis of Maxwell's equations is complicated. It is therefore to textbooks referred to electrodynamics, such as Jay Orear: Physics: Volume 2

## Application

The solar constant is about 1370 W / m². The result is a solar radiation pressure (german solar radiation pressure, SRP) on absorption of about 4.6 uPA. In total reflection it is twice as large. There are various ideas for a long time to use the radiation pressure with solar sails as propulsion for interplanetary spacecraft.

Conceivable in principle, is the driving force of spaceships produced by means of annihilation of matter and antimatter photons. Here, the radiation pressure of the emitted against the direction of flight of photons would be used.

The function of the light, however, is not based on the mills radiation pressure. This can be seen in the direction of rotation: the reflective side of the wing is exposed to a higher radiation pressure as the blackened, yet the mill rotates the other way around.

## Astrophysics

In astrophysics, radiation pressure plays a significant role in explaining the dynamics of stars and interstellar clouds.

The tail of the comet is caused to a large extent by the radiation pressure, the components of the coma " blows away ".

• Optics
• Astrophysics
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