Lift - On a body in a flowing fluid: the component of the force exerted by the fluid on the body at right angles to the direction of flow.

Light - Visible electromagnetic radiation of wavelength about 4000 to 7500A or 0.4 to 0.75 μm, together with the invisible radiation whose wavelengths lie immediately on either side of this range.

Light, Absorption Coefficient For - The fraction of a beam of light which is removed by absorption on passing through unit thickness of a material, measured for very small thicknesses. It is the coefficient α in the expression e-αx for the fraction of light transmitted through thickness x.

Lightning - A large-scale spark discharge which is associated with charged water discharge which is associated with charged water droplets or ice particles in a thunder cloud. The discharge may be entirely within the cloud, and is then known is sheet lightning; or it may reach the ground, when it is known as forked .lightning. Ligntning  flashes also occur which penetrate the cloud but do not reach the ground. The main charge within the cloud is negative and the difference in potential between this and the ground is about 108 V. Lightning flashes to the ground contain several successive strokes, i.e. partial discharges, which may be broadly classified as leader (downward) and return (upward) strokes.

Lightning, Ball - A slowly moving ball of fire (up to about a foot in diameter) associated with a thunderstorm, which is said to vanish with a loud report. Accounts of such phenomena are numerous but their reliability has not yet been firmly established.

Light, Theories Of - (1) The wave theory: the classical theory which considers electromagnetic radiation to be a wave motion, and which explains such properties as interference and diffraction. (2) The quantum theory: the theory which considers electromagnetic radiation to consist of particles of energy (quanta), called photons, and which explains many of the interactions of such radiation with matter, such as the photoelectric effect and the Compton effect.

The two theories are to be regarded as complementary rather than contradictory and represent two aspects of electromagnetic radiation. The energy of a photon is equal to the frequency of the corresponding waves multiplied by Planck's constant.

Light, Total Reflection Of - The complete reflection of a beam of light at the interface between two media of refractive index and respectively (where the light is travelling in the medium of refractive index nl) when the angle of incidence exceeds the critical angle, given by sin-l n2/n1, and nl is greater than n2. Total reflection also occurs with sound, radio and X-rays and may be described as internal or external according to circumstances.

For example, for X-rays falling upon a solid, the total relection is external, since the refractive index is smaller in the solid than in air. For light totally reflected at the inside surface of a prism, for example, it is internal. In the latter case there is some evidence that the light actually penetrates the interface, or is propagated along it, before "reflection".

Light Year - The distance travelled by light, in one astronomical year. It is equal to about 9.46 x 1012 km, or 5.9 x 1012 miles.

Limit Gauge - A gauge applied to mechanical components to verify that the component size lies between the permitted high and low limits.

Linde Liquefaction Process - A method of gas liquefaction using the Joule-Thomson effect. After expansion the gas is returned to the compressor through a counter-current heat exchanger in which the incoming gas is cooled.

Lindemann Electrometer - A form of quadrant electrometer which is insensitive to changes in level and which employs a needle as vane, whose movement is observed through a microscope for measurement.

Linear Accelerator - A straight-line accelerator for charged particles in which a number of electrodes are so arranged that, when a potential difference is applied at the proper radio frequency, the particles passing through them receive successive increments or energy. In the travelling wave linear accelerator, the particles are accelerated by the electric component of a travelling wave field set up a wave guide.

Linear Amplifier - An amplifier in which the output amplitude is directly proportional to the input amplitude. The term  generally refers to a pulse amplifier.

Linear Energy Transfer: Let - The average energy locally imparted to a medium per unit path by a charged ionizing particle of specified energy. It may be considered as a measure of the stopping power of that medium for .that particle. The concept is used mainly in radiation protection and radiation chemistry.

Linear Motor - An electromagnetic machine of the induction type, designed to produce force or motion in a straight line.

Line Frequency - Of a television system: the number of lines scanned per second by the cathode ray beam. It is a product of the number of lines in the picture and the number of times per second that the picture is scanned.

Line of Force - An imaginary line in an electric or magnetic field, the tangent to which, at a given point, represents the direction of the field at that point.

Line of Magnetic Induction - One of a series of lines which are thought of as passing from a magnetized body into the air at a north pole, entering the body again at the south pole, and returning through the body to the north pole to form a closed loop. This concept forms the basis of several definitions in the field of magnetism.

Line Voltage - The voltage between two line conductors of a polyphase a.c. system.

Linkage Mechanism - A restricted. form of mechanism for the transmission of motion. It consists of a series of rigid members joined together with constraints so that motion can be both amplified and redirected.

Liquid -   Describes that state or phase in which matter ultimately assumes the shape of its containing vessel, under gravitational . forces, up to a definite level- the liquid surface.

Liquid Degeneracy - The process by which a liquid, cooled below certain temperature, loses its entropy of liquid disorder without going into the solid state.

Lissajous Figures - Patterns which arise from the combination of two simple harmonic motions at right angles to each other. They can be produced in a cathode-ray osilloscope, for example, by supplying harmonically related voltages to the deflection plates; and may be used for comparing the frequencies and phases of the two motions.

Litre - The unit of volume in MKS and SI units. It is equal by defintion to 1 dm3 . It was formerly defined as the volume of 1 kg of pure water at 4°C which led to a value of the litre of 1.000028 dm3. This was rescined in 1964 by the Conference Generale des Poids et Mesures, which defined the litre as 10-3 m3 (1dm3) exactly, with the recommendation that neither the word litre nor its symbol “1” should be used to express results of high precision, to avoid confusion.

Littoral Current - The current along the seashore (or littoral zone).It causes intense marine abrasion.

Littrow Mirror - A plane mirror mounted nearly normal to the   dispersed beam emerging from the prism in a prism spectrometer.

Lobe - That portion of the overall radiation pattern of an aerial which is contained within a region bounded by directions  of minimum intensity. The term is sometimes used to describe the radiation within this region.

Lobe Switching: Beam Switching - In radar,a method of determining the direction of a target  by successive comparisons of the signals corresponding to two or more beam directions differing slightly from the target direction.

Lodestone - The first known magnet, consisting or the mineral magnetite. Fe3O4.

Logic Circuit - A circuit producing discrete output signals from discrete input signals, these signals being connected by specific relationhips, the most elementary of which are the Boolean "logical" relationships and, or and not.

Longitude. That coordinate of a point on the surface of a sphere or spheroid which specifies the azimuth of the point in the equatorial plane.

Loop, In An Electrical Circuit. A closed path around which actual or hypothetical currents can flow.

Lorentz Contraction. A hypothesis put forward by Fitzgerald to account for the null result of the Michelson-Morley experiment, which states that a body moving with velocity  v is contracted in the direction of motion by the factor √(1 – v2/c2), where C is the speed of light. It. is also known as the Lorentz Fitzgerald contraction and the Fitzgerald-Lorentz contraction.

Lorentz Factor (Crystal Analysis). A factor which occurs in expressions for the intensity of reflection of X-rays or neutrons by crystal planes. It takes account of the effect on the intensity of the orientation  of a crystal plane or the length of time (e.g. during the rotation of a single crystal) that the plane is in a reflecting position.

Lorentz Force. The force acting on a charge and current in a magnetic field.
Lorentz-Lorenz Formula. A formula relating the refractive index, n,   and the density q, of a gas. For a given wavelength and state of aggregation it may be  written as n2 – 1/n2  + 2= q x constant. For small  changes in n it leads to the Gladstone Dale law. It can also be applied to the variation of dielectric constant with density, when it    leads to the Clausius Mosotti  equation.

Lorentz Transformation. This gives the relationship,  according to the special theory of relativity,  between the description of an event in one frame of reference and that of the same event in a second frame of reference which is moving with a uniform velocity with respect to the first.

Lorentz Unit. A unit of frequency in  terms of which Zeeman splitting may be expressed. It is equal to eH/4πmc2, where e/m is the charge to mass ratio of the electron, H is the magnetic field strength and C the speed of light in a vacuum.