After careful observation, it was found that both energy and momentum were 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 reflecting from materials had decreased energy and correctly interpreted this as being due to the scattering of the X-ray photons by electrons. Some of the earliest direct experimental evidence of this came from the scattering of X-ray photons by electrons in substances, a phenomenon discovered by American physicist Arthur H. Momentum is conserved in quantum mechanics, just as it is in relativity and classical physics. Momentum, The Compton Effect, and Solar Sails The blue tail is caused by the solar wind, a stream of plasma consisting primarily of protons and electrons evaporating from the corona of the Sun. 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. Why would this be the case? The evidence indicates that the dust particles of the comet are forced away from the Sun when photons strike them. What most people do not know about the tails is that they always point away from the Sun rather than trailing behind the comet. Comet tails are composed of gases and dust evaporated from the body of the comet and ionized gas. Navy, via Wikimedia Commons)įigure 21.10 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. Figure 21.10 shows macroscopic evidence of photon momentum.įigure 21.10 The tails of the Hale-Bopp comet point away from the Sun, evidence that light has momentum. While it is strange to think of a massless particle exhibiting momentum, it is now a well-established fact within the scientific community. This is no clearer than when considering the photoelectric effect, where photons knock electrons out of a substance. Push them on the idea that the momentum equation relies on mass, while photons are massless particles.ĭo photons abide by the fundamental properties of physics? Can packets of electromagnetic energy possibly follow the same rules as a ping-pong ball or an electron? Although strange to consider, the answer to both questions is yes.ĭespite the odd nature of photons, scientists prior to Einstein had long suspected that the fundamental particle of electromagnetic radiation shared properties with our more macroscopic particles.
Ask them what equation they would use to calculate this momentum. They should answer yes, based on the photoelectric effect. Ask them whether photons demonstrate momentum.
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