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Nanometre (nm)

One nanometre is equal to one billionth of a metre.

Neutron

An uncharged particle that is similar in mass to a proton.

Non-locality

An influence is allowed to pass between two systems or particles instantaneously, exceeding the limit set by the speed of light, so that a cause at one place can produce an immediate effect at some distant location. Any theory that allows non-locality is called non-local. See locality.

Nucleus

The positively-charged mass at the heart of an atom. Initially believed to be made up only of protons, but later found to include neutrons. It contains virtually the entire mass of an atom but occupies a tiny fraction of its volume. Discovered in 1911 by Ernest Rutherford and his co-workers at Manchester University.

Observable

Any dynamical variable of a system or object that can, in principle, be measured. For example, the position, momentum, and kinetic energy of an electron are all observables.

Period

The time it takes for a single wavelength to pass a fixed point, and also the time required to complete one cycle of an oscillation or vibration. The period is inversely proportional to the frequency of a wave, vibration, or oscillation.

Periodic table

The arrangement of the elements according to their atomic number into rows and columns that displays their recurring chemical properties.

Photoelectric effect

The emission of electrons from a given metal surface when electromagnetic radiation above a certain minimum frequency (wavelength) strikes it.

Photon

The quantum of light characterised by the energy E=hv and momentum p=h/ where v and are the frequency and wavelength of the radiation. The name was introduced in 1926 by the American chemist Gilbert Lewis. See light-quanta.

Planck's constant (h)

A fundamental constant of nature with a value of 6.626×10-34 joule-seconds that lies at the heart of quantum physics. Because Planck's constant is not zero, it is responsible for chopping up, quantising, energy and other physical quantities in the atomic realm.

Potential energy

The energy that an object or system has by virtue of its position or state. For example, the height of an object above the earth's surface determines its gravitational potential energy.

Probability interpretation

The interpretation suggested by Max Born that the wave function allowed only the probability of finding a particle at a particular location to be calculated. It is part and parcel of the idea that quantum mechanics can generate only the relative probabilities of obtaining certain results from the measurement of an observable and cannot predict which specific result will be obtained on a given occasion.

Proton

A particle contained in the nucleus of an atom that carries a positive charge equal and opposite to that on an electron and that has a mass some 2,000 times that of the electron's.

Quantised

Any physical quantity that can only have certain discrete values is quantised. An atom has only certain discrete energy levels and its energy is therefore quantised. The spin of an electron is quantised since it can only be either +½ (spin up) or -½ (spin down).

QUANTUM

A term introduced by Max Planck in 1900 to describe the indivisible packets of energy that an oscillator could emit or absorb in his model as he tried to derive an equation that reproduced the distribution of blackbody radiation. A quantum of energy (E) comes in various sizes determined by E=hv, where h is Planck's constant and v is the frequency of the radiation. 'Quantum', more properly 'quantised', can be applied to any physical property of a microphysical system or object that is discontinuous, that can change only by discrete units.

Quantum jump

Also known as a quantum leap, it is the transition of an electron between two energy levels inside an atom or molecule due to the emission or absorption of a photon.

Quantum mechanics

The theory of physics of the atomic and subatomic realm that replaced the ad hoc mixture of classical mechanics and quantum ideas that emerged between