![]() But on a quantum-mechanical scale, especially for high-energy photons interacting with small masses, photon momentum is significant. This is borne out by the fact that it takes far less energy to give an electron the same momentum as a photon. A more massive particle with the same momentum would have an even smaller velocity. An electron with the same momentum has a 1460 m/s velocity, which is clearly nonrelativistic. Even if we have huge numbers of them, the total momentum they carry is small. Which is about five orders of magnitude greater. (See (Figure)) He won a Nobel Prize in 1929 for the discovery of this scattering, now called the Compton effect, because it helped prove that photon momentum is given by Energy and momentum are conserved in the collision. This phenomenon could be handled as a collision between two particles-a photon and an electron at rest in the material. ![]() Around 1923, Compton observed that x rays scattered from materials had a decreased energy and correctly analyzed this as being due to the scattering of photons from electrons. Some of the earliest direct experimental evidence of this came from scattering of x-ray photons by electrons in substances, named Compton scattering after the American physicist Arthur H. Momentum is conserved in quantum mechanics just as it is in relativity and classical physics. We expect particles with mass to have momentum, but now we see that massless particles including photons also carry momentum. Not only is momentum conserved in all realms of physics, but all types of particles are found to have momentum. Gas atoms and molecules in the blue tail are most affected by other particles of radiation, such as protons and electrons emanating from the Sun, rather than by the momentum of photons. Evidently, photons carry momentum in the direction of their motion (away from the Sun), and some of this momentum is transferred to dust particles in collisions. The dust particles recoil away from the Sun when photons scatter from them. Comet tails are composed of gases and dust evaporated from the body of the comet and ionized gas. What most people do not know about the tails is that they always point away from the Sun rather than trailing behind the comet (like the tail of Bo Peep’s sheep). (Figure) shows a comet with two prominent tails. The blue ionized gas tail is also produced by photons interacting with atoms in the comet material. Particles of dust are pushed away from the Sun by light reflecting from them. Dust emanating from the body of the comet forms this tail. The tails of the Hale-Bopp comet point away from the Sun, evidence that light has momentum. This book is archived and will be removed July 6, 2022.
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