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### PHYSICS DICTIONARY

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Laboratory Frame of Reference

Frame of reference, the centre of the coordinate system which lies on the laboratory in which experiment is done.

or

The frame of reference in which laboratory measurement devices are at rest is usually referred as laboratory frame of reference.

Lambda Particle

It is a elementary particle, which is neutral and discovered in cloud chamber experiments, having life time of order 10-10 sec and has rest energy of about 1115 MeV. It doesn’t conserve parity and has spin- 1/2. It belongs to class of Hyperons.

Lambda Point

When Helium is cooled to a critical temperature of 2.17 K (called its lambda point), a remarkable discontinuity in heat capacity occurs, the liquid density drops, and a fraction of the liquid becomes a zero viscosity "superfluid". Super fluidity arises from the fraction of helium atoms which has condensed to the lowest possible energy. In simple terms, the Lambda point is the temperature below which normal fluid Helium (Helium-I) transitions to super fluid Helium-II

Lambert

The unit of brightness or luminance in CGS system, equivalent to brightness of a perfectly diffusing surface that emits or reflects one Lumen per square centimeter. 1Lambert (L) = 1/π Candela/cm2.

Lambert’s Law of Absorption

Each layer of equal thickness absorbs an equal fraction of light which traverses it.

Lambert’s Law of Illumination

The illumination of a surface on which light falls normally from a point source is inversely proportional to square of distance of surface from source. If the normal to surface makes an angle with direction of rays, illumination is proportional to cosine of that angle.

Lambert’s law

Law which describes the attenuation of intensity of electromagnetic wave inside a medium is called as Lambert’s law.

Laminar Flow

It is type of fluid flow in which the fluid travels in regular paths. In laminar flow, sometimes called streamline flow, the velocity, pressure and other flow properties at each point in the fluid remain constant.

Laplace Equation

It is a mathematical equation which gives the divergence of the gradient of a function. Laplace’s equation is a  second-order partial differential equation widely useful in physics because its solutions (known as harmonic functions) occur in problems of electrical, magnetic, and gravitational potentials, of steady-state temperatures, and of Hydrodynamics. The equation was discovered by the French mathematician and astronomer Pierre-Simon Laplace (1749–1827).

Larmor’s Precession

When an atom is placed in an external magnetic field, the electron orbit precesses about the direction of external magnetic field. There will be change in angular velocity of electron without any change in form of orbit.

LASER

It is acronym for Light Amplification by Stimulated Emission of Radiation. It is that device which produces an intense, monochromatic, coherent and unidirectional light beam is obtained. It was discovered in 1950, by Town & Shallow. LASER produces a light beam in which all the waves are of same frequency.

Latent Heat (Heat of Transformation)

The amount of energy per unit mass that must be transferred as heat when a sample completely undergoes phase transition.

(or)

The amount of heat needed to change the state of a given substance without any change in temperature.

Latent Heat of Fusion

The quantity of heat necessary to change one gram of solid to liquid without change of temperature, measured as calories per gram.

Latent Heat of Sublimation

The heat absorbed by one gram or unit mass of a substance in the process of changing from a solid to a gaseous state, at a constant temperature and pressure.

Latent Heat of Vaporization

The quantity of heat necessary to change one gram of liquid to vapor without change of temperature, measured as calories per gram.

Lateral Chromatic Aberration

The difference in length of red and violet images for an object at finite distance is called lateral chromatic vibration.