Photoelectric Effect, Lasers and Superconductivity Notes

Three of the best questions in modern physics come from one idea: that light and matter behave in ways nineteenth-century physics could not explain. The photoelectric effect forced physicists to accept that light arrives in packets. The laser used the same quantum picture to make light of a purity nature never produces. Superconductivity showed that electrical resistance, which seems unavoidable, can vanish altogether. General awareness papers test all three with definition questions, discoverer questions and application questions, and this chapter arranges the facts in that order.

The photoelectric effect

When light of a suitable frequency falls on a metal surface, electrons are ejected from it. The effect was observed by Heinrich Hertz in 1887, while he was studying electric sparks, and was investigated further by Hallwachs and Lenard. The ejected electrons are called photoelectrons and the current they produce is the photoelectric current. Four features of the effect defeated the wave theory of light. First, emission happens only above a certain minimum frequency, called the threshold frequency, however bright the light may be. Second, the maximum kinetic energy of the photoelectrons depends on the frequency of the light and not at all on its intensity. Third, the number of photoelectrons, and hence the photocurrent, depends on the intensity. Fourth, emission is instantaneous, with no measurable time lag.

Einstein's explanation

In 1905 Albert Einstein explained the effect by treating light as a stream of quanta, later called photons, each carrying energy equal to Planck's constant multiplied by the frequency. A photon gives all its energy to one electron. Part of it is spent in freeing the electron from the metal, and the rest appears as kinetic energy. That is Einstein's photoelectric equation: the energy of the photon equals the work function plus the maximum kinetic energy of the photoelectron. The work function is the minimum energy needed to pull an electron out of the metal, and it equals Planck's constant times the threshold frequency. The stopping potential is the negative voltage that just stops the fastest photoelectron, so it measures the maximum kinetic energy. Einstein received the Nobel Prize in Physics for 1921 for this work, and Robert Millikan's careful experiments confirmed the equation and gave a good value of Planck's constant, about 6.63 into ten to the power minus thirty-four joule second.

Photoelectric applications

Metals with a low work function are the useful ones, so caesium, potassium and sodium respond to ordinary visible light while zinc and magnesium need ultraviolet. Photocells built on the effect work as automatic switches for street lights, as burglar alarms, as light meters in cameras, as the readers of film sound tracks and as detectors in counting and sorting machines. Solar cells use a related photovoltaic effect to turn light directly into electricity.

Lasers

The word laser is an abbreviation for light amplification by stimulated emission of radiation. Stimulated emission, in which a photon passing an excited atom makes it drop to a lower level and emit a second identical photon, was predicted by Einstein in 1917. Charles Townes built the first maser, working with microwaves, in 1954, and Theodore Maiman built the first working laser, a ruby laser, in 1960. A laser needs three things: an active medium, a pumping source that raises atoms to an excited state, and a pair of mirrors forming a resonant cavity. The essential condition is population inversion, in which more atoms are in the higher energy level than in the lower one, and it is made possible by a metastable state in which atoms can wait.

Lasers are grouped by their active medium. Solid state lasers include the ruby laser, whose active centres are chromium ions in aluminium oxide, and the neodymium YAG laser. Gas lasers include the helium-neon laser, which gives the familiar red beam, and the carbon dioxide laser, used for cutting and welding metal. Semiconductor diode lasers are the small, cheap ones in optical drives and laser pointers. Applications are everywhere: reading compact discs and barcodes, fibre-optic communication, laser printing, holography, which Dennis Gabor invented and for which he received the Nobel Prize in 1971, distance measurement and LIDAR, and in medicine eye surgery, the removal of tumours and bloodless cutting.

Superconductivity

Superconductivity is the complete disappearance of electrical resistance in certain materials when they are cooled below a particular temperature, called the critical temperature or transition temperature. It was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes, who found that mercury lost all resistance below about 4.2 kelvin, close to the boiling point of liquid helium. A current once started in a superconducting ring keeps flowing without any applied voltage, because no energy is lost as heat.

A superconductor also expels magnetic field from its interior, behaving as a perfect diamagnet. This is the Meissner effect, reported in 1933, and it is the reason a magnet can be made to float above a cooled superconductor. If the magnetic field is raised beyond a critical value, or the current beyond a critical current, superconductivity is destroyed. The microscopic explanation is the BCS theory, given in 1957 by Bardeen, Cooper and Schrieffer, in which electrons move in bound pairs called Cooper pairs that are not scattered by the lattice. In 1986 Bednorz and Muller found superconductivity in a ceramic oxide at a much higher temperature, and the yttrium barium copper oxide compound that followed works above 77 kelvin, so it can be cooled with liquid nitrogen instead of liquid helium. Superconducting magnets are used in magnetic resonance imaging machines, in particle accelerators, in magnetically levitated trains and in the extremely sensitive magnetometers called SQUIDs.