Lande G-Factor: Lande Splitting Factor - A factor of proportionality, g, introduced by Lande to describe the anomalous Zeeman effect for which the change in electronic energy level induced by a magnetic field (the "splitting") is given by gβHM, where βis the Bohr magneton (eh/4πmc where e is the electronic charge, h is Planck's constant, m is the electron rest mass and c is the speed of light), H is the magnetic field and M the magnetic quantum number. The value of g depends on the orbital angular momentum and the spin angular momentum quantum numbers, and ranges from 1, for pure orbital momentum, to 2, for pure spin momentum.

Lane Law - States that if a star contracts, its internal temperature rises.

Langmuir Law - States that, in a thermionic diode with space charge limited current, the anode current density is proportional to the 3/2 power of the anode-cathode voltage.

Laporte Selection Rule - For atomic spectra: states that for electric dipole radiation, terms for which the sum of the azimuthal quantum numbers of the individual electrons are even combine only with those terms for which the sum is odd, and vice versa.

Lapse Rate - The rate of decrease of temperature with height in the troposphere. and mesosphere. When the temperature increases with height the lapse rate is said to be negative and we speak of a temperature inversion. The dry adiabatic lapse rate is the rate at which an element of unsaturated air ascending (descending) adiabatically is cooled (warmed). The saturated adiabatic lapse rate is the corresponding rate for saturated air. Values of the lapse rate are in the region of 1 °C/100

Larmor Precession - The precessional motion of the orbit of a charged particle which is subjected to a magnetic field. The precession occurs about the direction of the field. For an electron revolving about a nucleus the angualr velocity of the Larmor precession is given by eh/2mc, where e is the charge, m is the mass, c the speed of light and H the magnetic field strength.

Larmor Precession Frequency - The frequency of the Larmor precession, also known as the gyromagnetic frequency.

Laser - An acronym for "Light amplification by stimulated emission of radiation". A laser uses the maser technique in the optical region, and has been termed an optical maser. However, sub-millimetre lasers are now also in use: The emphasis in the optical region, in contrast with that in the microwave region, has been in the development of the laser oscillator, which provides a source of light that is completely phase-coherent; and an intense beam may be provided by the use of resonance techniques. The laser effect has been observed in solids, liquids, and gases. See also: Maser. Optical pumping.

Latent Heat - The energy associated with   change of physical state of any substance at constant temperature and pressure under equilibrium conditions. It includes heats of fusion, vaporization! sublimation and transition (e.g. from one crystal form to another). and the heats of their reverse processes. It does not in general include heats of absorption. adsorption, desorption, neutralization, solution, dilution, mixing, expansion compression, dissociation or combination, and chemical reactions.

Latitude - That coordinate of a point on the surface of a sphere or spheroid which specifies the angular elevation above the equator of a radius or normal passing through the ,point. See the following terms for particular examples.

Lattice - (1) Of a crystal: a term loosely, used for space lattice. (2) Of a nuclear reactor: an array of nuclear fuel and other materials arranged according to a regular pattern.

Lattice Conduction of Heat - One of the two modes of heat conduction in a metal, the other being electron conduction. Lattice conduction arises from the motion of atoms vibrating about their equilibrium positions and the heat may be considered to be transmitted by the motion of vibrational quanta or phonons.

Lattice Constants: Lattice Dimensions: Lattice Parameters - Terms loosely used to denote unit cell dimensions. They specify the size and shape of the unit cell of a crystal structure in terms of the cell edges and their angles of intersection.

Laue Equations - A set of three equations governing the diffraction of X-rays which must be satisfied to permit reinforcement of the contributions scattered from atoms at successive equivalent points of a crystal along each of its coordinate axes. This set of equations represents an alternative to the Bragg law as a method of expressing the conditions for selective reflection. The Laue equations also hold for the diffraction of electrons. neutrons, etc.

Laue Method; Laue Photography - The examination of the diffracted beams which are produced for any arbitrary setting of a stationary crystal from a beam of "white" radiation (i.e. a beam containing a wide continuous range of wavelengths).Each set of crystal planes selects the wavelength that will satisfy the Laue equations and produces a diffraction “spot” on a photographic film.

Leading Current - In an a.c. circuit with capacitive (or negative) total reactance: the current which reaches its maximum value at an earlier instant in the cycle than the impressed voltage.

Leakage Current - The electrical current which results from charge leaking along the surface or through the body of an insulator. It is surface or through the body of an insulator. It is usually negligible except for a diety, moist or cracked insulator.

Least Action, Principle Of - States that a conservative dynamical system in passing, from one configuration to another does so in such a way that the action of the system remains stationary. This stationary value could be a maximum but is usually found to be a minimum.

Leed - An acronym for "Low energy electron diffraction".

Length, Standards Of - Formerly signified the Imperial standard yard held in London and the international prototype metre held at Sevres. These are line standards whose value is defined by the distance between two lines on a stable metal. The internationally agreed standard of length is now, however, based on the radiation emitted by the krypton isotope 86Kr, the metre being defined as the length equal to 1650763.73 wavelengths in vacuum of the radiation (orange in colour) corresponding to the transition between the levels 2p10 and 5d5 of that isotope, the radiation being emitted in the absence of any perturbing effects which cause a wavelength change, such as Doppler pressure, or Stark effects. In addition the inch (being 1/36y)is legally 2.54 x 10-2 m.

Length, Units Of - Two units are in common use: the yard, the unit of the British system; and the metre, the unit of the Metric system. The Imperial standard yard has now been superseded by international agreement whereby one inch (1/36yd) being is equal to 2.54 x 10-2 m. exactly. See also: Metre. Toise. Yard.

Lens - (1) Optical: a portion of a homogeneous refracting medium bounded by two spherical surfaces and with an axis of symmetry­- the principal axis-which passes through the centre of curvature of the two surfaces. A beam of parallel light will converge or diverge in passing through the lens according as the main surface is convex or concave. (2) Magnetic: an arrangement of electric or magnetic fields designed to focus an electron beam. See: Electron lens. (3) Dielectric: a lens of dielectric material used for refraction at radio. frequency. (4) Of the eye: a fibrous lens situated between the aqueous and vitreous humours and behind the iris. It is often known as the crystalline lens.

Lens, Achromatic - A lens designed to minimize chromatic aberration. In its simplest form it is composed of two lenses, one convergent and the other divergent, made of glasses having different dispersive powers, the ratio of their focal lengths being equal to the ratio of their dispersive powers.

Lens Aplanatic - A lens in whose construction use is made of aplanatic point, whereby it possesses the property of giving a sharp image for rays making large angles with the axis.

Lens, Apochromatic - A compound lens that is sensibly free from chromatic errors, from spherical aberration for two wavelengths, and from central coma for one wavelength.

Lens, Aspherical - A lens whose surface is made not quite spherical in order to reduce aberrations. The process of treating a spherical surface (e.g. by polishing) so as to remove aberrations is known as figuring and a lens so treated is known as a figured lens.

Lens, Cardinal Points Of - Six points of a thick lens which are used in determining image positions. They consist of two focal points, two principal points and two nodal points defined as follows. The first focal point is the point on the axis at which a point object will form an image at infinity. The second focal point is the point at which a parallel beam of light parallel to the axis will form an image. The two principal points are the intersections with the axis of the two principal planes (conjugate planes of unit positive lateral magnification). The two nodal points are conjugate points on the axis having unit positive angular magnification.

Lens, Compound - A lens consisting of a combination of several simple lenses. usually made from different types of glass. Such lenses are employed to reduce the various aberrations exhibited by simple lenses.

Lens, Contact - A lens worn underneath the eyelid and in contact with part of the eyeball. It is usually made of plastic.