The propagation vector shows the direction of the photon’s linear momentum vector. Vector k → k → is called the “wave vector” or propagation vector (the direction in which a photon is moving). In Equation 6.22, ℏ = h / 2 π ℏ = h / 2 π is the reduced Planck’s constant (pronounced “h-bar”), which is just Planck’s constant divided by the factor 2 π. According to the theory of special relativity, any particle in nature obeys the relativistic energy equation For example, how can we find the linear momentum or kinetic energy of a body whose mass is zero? This apparent paradox vanishes if we describe a photon as a relativistic particle. From the point of view of Newtonian classical mechanics, these two characteristics imply that a photon should not exist at all. In a vacuum, unlike a particle of matter that may vary its speed but cannot reach the speed of light, a photon travels at only one speed, which is exactly the speed of light. Unlike a particle of matter that is characterized by its rest mass m 0, m 0, a photon is massless. This idea proved useful for explaining the interactions of light with particles of matter. A beam of monochromatic light of wavelength λ λ (or equivalently, of frequency f) can be seen either as a classical wave or as a collection of photons that travel in a vacuum with one speed, c (the speed of light), and all carrying the same energy, E f = h f. Beyond 1905, Einstein went further to suggest that freely propagating electromagnetic waves consisted of photons that are particles of light in the same sense that electrons or other massive particles are particles of matter. Two of Einstein’s influential ideas introduced in 1905 were the theory of special relativity and the concept of a light quantum, which we now call a photon. Describe how experiments with X-rays confirm the particle nature of radiation.By the end of this section, you will be able to:
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