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29.4 Photon Momentum
A photon interacts as a unit, rather than as an extensive wave.
Massive quanta act like particles because they are the smallest units of matter.
Particles carry both energy and momentum.
Despite the fact that there is no mass, there is evidence that radiation carries momentum.
It's a well-established fact that there's a lot of momentum in the photon.
The photoelectric effect suggests photon momentum, where electrons are knocked out of a substance.
The tails of the Hale-Bopp comet point away from the Sun.
This tail is formed by dust from the comet.
Light reflecting from dust pushes it away from the Sun.
The blue ionized gas tail is produced by interacting with atoms in the comet material.
There is a comet with two tails.
Most people don't know that the tails point away from the Sun rather than behind the comet.
Dust is evaporated from the comet's body of ionized gas.
The dust particles move away from the Sun.
Some of the momentum from the Sun is transferred to dust particles in a collision.
The blue tail's gas atoms and molecules are most affected by other particles of radiation, such as protons and electrons coming from the Sun.
All types of particles are found to have momentum.
We expect particles with mass to carry momentum, but now we see that massless particles do the same.
It is the same thing in quantum mechanics as it is in classical physics.
The earliest direct experimental evidence of this came from scattering of x-ray photons by electrons in substances.
The x rays that were scattered from materials had a decreased energy and were analyzed by Compton as a result of the scattering of photons from electrons.
The phenomenon could be handled as a collision between two particles.
The collision has energy and momentum.
The name of the effect is the scattering of a photon by an electron.
The reduction of both energy and momentum for the scattered photon is a result of energy and momentum being conserved.
The effect was verified by Compton.
The photon momentum is very small.
For this reason, we don't usually observe photon momentum.
Our mirrors don't recoil when light hits them.
x rays have a small wavelength and a relatively large momentum, interacting with the lightest of particles, the electron.
If the photon momentum is small, we can assume that an electron with the same momentum will be nonrelativistic, making it easy to find its velocity and energy from the classical formulas.
The classical expression will be used to find the velocity of an electron with this momentum.
It is five orders of greater magnitude.
It is indeed small.
The total momentum they carry is small even if we have a lot of them.
A 1460 m/s velocity is clearly nonrelativistic for an electron with the same momentum.
A bigger particle with the same momentum would have a smaller speed.
It takes less energy to give an electron the same strength as a photon.
On a quantum-mechanical scale, photon momentum is significant.
If there is enough photon momentum, it can have an effect if there is not enough to prevent the slow recoil of matter.
There are also proposals to build space sails that use huge low-mass mirrors to reflect sunlight.
The mirrors would start to recoil in the vacuum of space, which would allow them to take the craft from place to place in the solar system.
A Russian test model of this was launched in 2005, but did not make it into space due to a rocket failure.
A 40-m2 sail is what it will have.
The relationship between photon energy and photon momentum is similar to the one given for the total energy of a particle.
For a photon, note the validity of this relation.
It's equivalent to Compton's result.
Most detection systems talked about the particle-like properties of photons interacting with a sensitive area.
A change can be cascaded or recorded to form an image.
There is ongoing research into improving the efficiency of receiving photons, particularly by cooling detection systems and reducing thermal effects.
The photon is considered in the example 29.5.
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