Annual report of the Board of Regents of the Smithsonian Institution . X ray, after passingthrough the walls of the X-ray tube and perhaps a block of wood, mayeject a photoelectron from a metal plate placed on the far side. Thespeed of this photoelectron is then found to be almost as great asthat of the original cathode electron. WHAT IS LIGHT? COMPTON 221 The surprising nature of this phenomenon may be illustrated by-considering a similar event with water waves. Imagine two divingboards on opposite sides of a wide pond. A boy dives from one boardinto the water with a splash which sends ripple
Annual report of the Board of Regents of the Smithsonian Institution . X ray, after passingthrough the walls of the X-ray tube and perhaps a block of wood, mayeject a photoelectron from a metal plate placed on the far side. Thespeed of this photoelectron is then found to be almost as great asthat of the original cathode electron. WHAT IS LIGHT? COMPTON 221 The surprising nature of this phenomenon may be illustrated by-considering a similar event with water waves. Imagine two divingboards on opposite sides of a wide pond. A boy dives from one boardinto the water with a splash which sends ripples out over the the time they reach the second boy, who is swimming in the waterbeside the other diving board some distance away, these ripples aremuch too small to notice. We should be greatly surprised if theseinsignificant ripples should lift the second swimmer bodily from thewater and set him on his diving board. If, however, it is impossible for a water ripple to do such a thing itis just as impossible for an ether ripple, sent out when an electron dives. FiGUBE -The speed of the photoelectrons ejected from the metal plate at P is almost as great as thespeed of the cathode electrons which produce the X rays at the target T into the target of an X-ray tube, to jerk an electron out of a secondpiece of metal with a speed equal to that of the first electron. It was considerations of this kind which showed to Einstein thefutility of trying to account for the photoelectric effect on the basis ofwaves. He saw, however, that this effect might be explained if lightand X rays consist of particles. These particles are now commonlycalled photons. The picture of the X-ray experiment on thisview would be that when the electron strikes the target of an X-raytube, its energy of motion is transformed into a photon, that is, aparticle of X rays which goes with the speed of light to the second piece 222 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 2 9 of metal. Here the photon gives
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