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Arthur Compton discovered the Compton effect in the early 1920s. It shows an increase in the wavelength of X-rays (or gamma rays) when they are scattered by electrons. If the X-rays behaved as classical waves, it would be impossible to explain this effect, because there is no reason why the scattering should alter the wavelength. But Compton realized that it could be explained by treating the X-rays as a stream of particles, now known as photons. When a high-energy photon scatters off an electron, it loses energy, which is taken up by the electron. The loss of energy is equivalent to an increase in wavelength in line with Planck’s equation E = hf, where h is Planck’s constant.
This explanation, confirmed in a series of careful experiments, was clinching proof of the dual wave/particle nature of electromagnetic radiation, and instrumental in encouraging Louis de Broglie’s development of the idea of electrons as having the same kind of dual wave/particle nature.
In Young's version of this experiment, light is shone on to a screen, which has a small hole. After passing through this hole, the light arrives at a second screen, which has two holes in it. Light spreading out from the two holes in the second screen finally falls on a third screen, where the light shows a diffraction pattern of light and shade. This is not the way a stream of particles would behave, so if light were a stream of particles, you would expect to find just two bright patches on the detector screen, one behind each screen, not an interference pattern.
The exact same set up can be used to carry out the experiment using a beam of electrons instead of light. Electrons fired through a double slit experiment produce an interference pattern on the detector screen (in this case, a screen much like that of a television, where the arrival of each electron makes a single point of light). When considering what happens in this experiment you notice that an interference pattern is produced and therefore the electrons are travelling as waves, but the arrival of each electron at one particular place on the screen makes a single spot of light. This means that they are arriving as particles. The quantum entities travel like waves but arrive as particles.
It was then decided to try repeating the experiment firing the quantum entities (either photons or electrons) through the slits one at a time, and the pattern they make on the detector screen is allowed to build up gradually. Now, single particles are travelling one at a time through the experiment, and each makes a single spot on the screen. Each particle should only go through one or the other of the two holes, but as more and more spots build up on the screen, the pattern that emerges is the classic interference pattern for waves passing through both holes at once. The quantum entities not only seem to be able to pass through both holes at once, but to have an awareness of past and future so that each can ‘choose’ to make its own contribution to thee interference pattern, in just the right place to build the pattern up without destroying it.
The photoelectric effect is the release of electrons from a substance under the influence of light or other electromagnetic radiation. The effect has a key place in the history of the development of quantum physics because Albert Einstein explained it, in 1905, by treating light in terms of a stream of particles rather than as a wave. Although the implications were not fully appreciated at the time this was the first step towards the concept of wave/particle duality. It was for this work, not for either of his theories of relativity, that Einstein received the Nobel Prize.
The experiments, first conducted by J. J. Thomson and Philip Lenard at the end of the nineteenth century, used beams of light of a single colour (so all the light had the same frequency) shone onto metal surfaces. Using a bright light, there is more energy shining on each square centimetre of the surface of the metal than with a dim light. You might expect that with more energy available, the electrons knocked out of the metal surface by the light would be more energetic and travel faster. In fact, it turns out that as long as the frequency of the light stays the same, the energy of each electron emitted is the same; but when the light is brighter, more electrons are liberated. The electrons do, though, move faster if light with a higher frequency is shone on the metal.
Einstein explained this by taking the equation at the heart of Planck’s description of black body radiation, E = hf, and applying it to the electromagnetic radiation itself. He said that the photoelectric effect could be explained if light itself came in definite packets, or quanta, each with an energy hf, where h is Planck’s constant and f is the frequency of the radiation. It takes one light quantum to knock one electron out of the metal, and for a particular frequency all the light quanta have the same energy.
It was in 1909 that Rutherford suggested the experiment, actually carried out by Hans Geiger and Ernest Marsden, which led to the first nuclear model of the atom, which Rutherford announced in 1911.
Geiger and Marsden fired a beam of alpha particles at a thin sheet of metal foil. Most of the particles went straight through but occasionally one bounced back from the foil. The Thomson model could not explain this, because on that picture the foil would be uniformly made up of serried ranks of ‘plum-puddings’ touching each other. There would be no hard centres for the alpha particles to bounce off. So Rutherford developed a new picture of the atom, the Rutherford model, in which a tiny central nucleus (much smaller than one Ångström in diameter), which contains all the positive charge and almost all of the mass of the atom, is orbited by electrons in a way roughly analogous to the way the sun is orbited by the planets. On this picture, most of any seemingly solid object, including a sheet of foil, is empty space. An alpha particle would easily brush through he electron clouds in the gaps between nuclei, but occasionally an alpha particle would hit a nucleus more or less head on, and be deflected by a large angle.
British physicist, Charles Wilson, invented the cloud chamber, which he actually invented in order to study clouds, but soon realized could be used as a particle detector. The cloud chamber was the first type of detector to show the tracks of elementary particles, using the same principal as the way in which ‘vapour trails’ form behind high-flying aircraft. The first cloud chamber was a desktop-sized apparatus in which a glass chamber full of moist air was connected to a piston, which could be suddenly moved outward, lowering the pressure and causing mist (or cloud) to form in the chamber. The mist droplets grow on tiny particles of dust in the air (cloud condensation nuclei). But Wilson found that even when all the dust had been removed from the chamber, when the piston was rapidly moved out over a large distance, a very thin mist still formed in the chamber. He surmised that the droplets were condensing around electrically charged particles (ions), and proved this, early in 1896, by operating the cloud chamber alongside a source of X-rays and seeing it fill up with condensation as the X-rays passed through it ionising the atoms in the air inside the chamber.
Wilson did not develop the idea further until 1910, when he fired alpha and beta radiation through a cloud chamber and, for the first time, saw tracks of individual particles as thread-like clouds.