Compton scattering

According to classical physics, light is pure energy and has no mass. No mass implies no momentum since the classical definition of momentum is mass times velocity, p = mv. But the wave-particle duality for light allows photons to have no mass but still have momentum. Read the text aloud Show Misconceptions about the Crooke’s radiometer
Compton scattering experimentIn 1923, Arthur Compton conducted a series of experiments on light in which he directed x-rays to hit a graphite target. Compton measured the scattered x-rays at various angles. Although the incident x-rays had a single wavelength, at any given angle Compton found the reflected light to have two different wavelengths. One of the two wavelengths was the incident wavelength λ1, while the other was a slightly longer wavelength λ2. The longer wavelength radiation implied that the energy of this second reflection was lower. Compton explained the shift in energy by postulating that the incident radiation was not a wave but rather a collection of particles called photons, each with energy E = hf. Read the text aloud
Why Compton's experiment showed that a photon has momentumCompton deduced that the shift in frequency came from an elastic collision between a photon and an electron in a carbon atom in the graphite. Compton calculated the energy of the scattered photon by applying the laws of energy and momentum conservation: Read the text aloud
momentum before = momentum after
energy before = energy after
Show Calculating the Compton shift
Compton scattering dataThe difference Δλ between the scattered wavelength λ2 and the incident wavelength λ1 increases as the detector angle increases. This result cannot be explained by the wave theory of light. Compton concluded that the photon “carries with it directed momentum as well as energy.” Photons of light conserve energy and momentum in their interactions, much like particles. A complete description of radiation has both wave aspects, to explain interference and diffraction, and particle aspects, to explain how photons interact with matter. Read the text aloud Show Photons interact with free and bound electrons

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