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Extremely short-wavelength electromagnetic radiation. It is the most penetrating of the three types of radiation emitted by radioactive substances.

Ground state

The lowest energy state that an atom can possess. All other atomic states are called excited states. The lowest energy state of a hydrogen atom corresponds to its electron occupying the lowest energy level. If it occupies any other energy level, the hydrogen atom is in an excited state.

Harmonic oscillator

A vibrating or oscillating system whose frequency of vibration or oscillation does not depend on the amplitude.

Hidden variables

An interpretation of quantum mechanics based on the belief that the theory is incomplete and that there is an underlying layer of reality that contains additional information about the quantum world. This extra information is in the form of the hidden variables, unseen but real physical quantities. The identification of these hidden variables would lead to exact predictions for the outcomes of measurements and not just probabilities of obtaining certain results. Its adherents believe that it would restore a reality that exists independently of observation, denied by the Copenhagen interpretation.

Infrared radiation

Electromagnetic radiation with wavelengths longer than visible red light.

Interference

This is a characteristic phenomenon of wave motion in which two waves interact. Where two wave troughs or crests meet, they coalesce to produce a new, bigger trough or crest; this is known as constructive interference. But where a trough meets a crest or vice versa, they cancel each other out, a process called destructive interference.

Isotopes

Different forms of the same element that have the same number of protons in the nucleus, i.e. that share the same atomic number, but each having a different number of neutrons. For example, there are three forms of hydrogen with their nuclei containing zero, one, and two neutrons respectively. All three have similar chemical properties but different masses.

Joule

A unit of energy used in classical physics. A 100-watt light bulb converts 100 joules of electrical energy per second into heat and light.

Kinetic energy

Energy associated with the motion of an object. A stationary object, planet or particle has no kinetic energy.

Light

The human eye can detect only a small portion of all electromagnetic waves. These visible wavelengths of the electromagnetic spectrum range between 400nm (violet) and 700nm (red). White light is made up of red, orange, yellow, green, blue, indigo, and violet light. When a beam of white light is passed through a glass prism, these different strands of light are unpicked and form a rainbow band of colours called a continuum or continuous spectrum.

Light-quanta

The name first used by Einstein in 1905 to describe particles of light, later renamed photons.

Locality

The requirement that a cause and its effects occur at the same place, that there is no action at a distance. If an event A is the cause of another at B, there must be enough time between the two to allow a signal travelling at the speed of light from A to reach B. Any theory which has locality is called local. See non-locality.

Matrices

Arrays of numbers (or other elements such as variables) with their own rules of algebra, matrices are extremely useful for expressing information about a physical system. An n×n square matrix has n columns and n rows.

Matrix mechanics

A version of quantum mechanics discovered by Heisenberg in 1925 and then developed in conjunction with Max Born and Pascual Jordan.

Matter wave

When a particle behaves as though it has a wave character, the wave representing it is called a matter wave or a de Broglie wave. See deBroglie wavelength.

Maxwell's equations

A set of four equations derived by James Clerk Maxwell in 1864 that unified and described the disparate phenomena of electricity and magnetism as a single entity – electromagnetism.

Momentum (p)

A physical property of an object that is equivalent to its mass times its