Kerr effect (J. Kerr; 1875)

The ability of certain substances to differently refract lightwaves whose vibrations are in different directions when thesubstance is placed in an electric field.

Kirchhoff's law of radiation (G.R. Kirchhoff)

The emissivity of a body is equal to its absorptance at the sametemperature.

Kirchhoff's rules (G.R. Kirchhoff)

The loop rule. The sum of the potential differences encountered in a round trip around any closed loop in a circuit is zero.

The point rule. The sum of the currents toward a branch point is equal to the sum of the currents away from the same branch point.

Kohlrausch's law (F. Kohlrausch)

If a salt is dissolved in water, the conductivity of the solutionis the sum of two values -- one depending on the positive ions andthe other on the negative ions.

Lambert's laws (J.H. Lambert)

Lambert's first law. The illuminance on a surface illuminated by light falling on it perpendicularly from a point source is proportional to the inverse square of the distance between the surface and the source.

Lambert's second law. If the rays meet the surface at an angle, then the illuminance is also proportional to the cosine of the angle with the normal.

Lambert's third law. The luminous intensity of light decreases exponentially with the distance that it travels through an absorbing medium.

Landauer's principle

A principle which states that it doesn't explicitly take energy tocompute data, but rather it takes energy to erase any data,since erasure is an important step in computation.

Laplace's equation (P. Laplace)

For steady-state heat conduction in one dimension, the temperaturedistribution is the solution to Laplace's equation, which statesthat the second derivative of temperature with respect todisplacement is zero.

Laue pattern (M. von Laue)

The pattern produced on a photographic film when high-frequencyelectromagnetic waves (such as x-rays) are fired at a crystallinesolid.

Laws of conservation

A law which states that, in a closed system, the total quantity ofsomething will not increase or decrease, but remain exactly thesame. For physical quantities, it states that something canneither be created nor destroyed.

The most commonly seen are the laws of conservation of mass-energy (formerly two conservation laws before A. Einstein), ofelectric charge, of linear momentum, and of angular momentum.There are several others that deal more with particle physics,such as conservation of baryon number, of strangeness, etc., whichare conserved in some fundamental interactions but not others.

Law of reflection

For a wavefront intersecting a reflecting surface, the angle ofincidence is equal to the angle of reflection.

Laws of black hole dynamics

First law of black hole dynamics. For interactions between black holes and normal matter, the conservation laws of total energy, total momentum, angular momentum, and electric charge, hold.

Second law of black hole dynamics. With black hole interactions, or interactions between black holes and normal matter, the sum of the surface areas of all black holes involved can never decrease.

Laws of thermodynamics

First law of thermodynamics. The change in internal energy of a system is the sum of the heat transferred to or from the system and the work done on or by the system.

Second law of thermodynamics. The entropy -- a measure of the unavailability of a system's energy to do useful work -- of a closed system tends to increase with time.

Third law of thermodynamics. For changes involving only perfect crystalline solids at absolute zero, the change of the total entropy is zero.

Zeroth law of thermodynamics. If two bodies are each in thermal equilibrium with a third body, then all three bodies are in thermal equilibrium with each other.

Lawson criterion (J.D. Lawson)

A condition for the release of energy from a thermonuclearreactor. It is usually stated as the minimum value for theproduct of the density of the fuel particles and the containmenttime for energy breakeven. For a half-and-half mixture ofdeuterium and tritium at ignition temperature, n_G t is between10¹⁴ and 10¹⁵ s/cm³.

Le Chatelier's principle (H. Le Chatelier; 1888)

If a system is in equilibrium, then any change imposed on thesystem tends to shift the equilibrium to reduce the effect of thatapplied change.

Lenz's law (H.F. Lenz; 1835)

An induced electric current always flows in such a direction thatit opposes the change producing it.

Loschmidt constant; Loschmidt number; N_L

The number of particles per unit volume of an ideal gas atstandard temperature and pressure. It has the value 2.68719^.10²⁵ m^-3.

Lumeniferous aether

A substance, which filled all the empty spaces between matter,which was used to explain what medium light was "waving" in. Nowit has been discredited, as Maxwell's equations imply thatelectromagnetic radiation can propagate in a vacuum, since theyare disturbances in the electromagnetic field rather thantraditional waves in some substance, such as water waves.

Lyman series

The series which describes the emission spectrum of hydrogen whenelectrons are jumping to the ground state. All of the lines arein the ultraviolet.

Mach's principle (E. Mach; 1870s)

The inertia of any particular particle or particles of matter isattributable to the interaction between that piece of matter andthe rest of the Universe. Thus, a body in isolation would have noinertia.

Magnus effect

A rotating cylinder in a moving fluid drags some of the fluidaround with it, in its direction of rotation. This increases thespeed in that region, and thus the pressure is lower.Consequently, there is a net force on the cylinder in thatdirection, perpendicular to the flow of the fluid. This is calledthe Magnus effect.

Malus's law (E.L. Malus)

The light intensity travelling through a polarizer is proportionalto the initial intensity of the light and the square of the cosineof the angle between the polarization of the light ray and thepolarization axis of the polarizer.

Maxwell's demon (J.C. Maxwell)

A thought experiment illustrating the concepts of entropy. Wehave a container of gas which is partitioned into two equal sides;each side is in thermal equilibrium with the other. The walls(and the partition) of the container are a perfect insulator.    Now imagine there is a very small demon who is waiting at thepartition next to a small trap door. He can open and close thedoor with negligible work. Let's say he opens the door to allow afast-moving molecule to travel from the left side to the right, orfor a slow-moving molecule to travel from the right side to the left, and keeps it closed for all other molecules. The net effectwould be a flow of heat -- from the left side to the right -- eventhough the container was in thermal equilibrium. This is clearlya violation of the second law of thermodynamics. So where did we go wrong? It turns out that information hasto do with entropy as well. In order to sort out the moleculesaccording to speeds, the demon would be having to keep a memory ofthem -- and it turns out that increase in entropy of the simplemaintenance of this simple memory would more than make up for thedecrease in entropy due to the heat flow.

Maxwell's equations (J.C. Maxwell; 1864)

Four elegant equations which describe classical electromagnetismin all its splendor. They are:

Gauss' law. The electric flux through a closed surface is proportional to the algebraic sum of electric charges contained within that closed surface.

Gauss' law for magnetic fields. The magnetic flux through a closed surface is zero; no magnetic charges exist.

Faraday's law. The line integral of the electric flux around a closed curve is proportional to the instantaneous time rate of change of the magnetic flux through a surface bounded by that closed curve.

Ampere's law, modified form. The line integral of the magnetic flux around a closed curve is proportional to the sum of two terms: first, the algebraic sum of electric currents flowing through that closed curve; and second, the instantaneous time rate of change of the electric flux through a surface bounded by that closed curve.

In addition to describing electromagnetism, his equations alsopredict that waves can propagate through the electromagneticfield, and would always propagate at the same speed -- these are electromagnetic waves.

Meissner effect (W. Meissner; 1933)

The decrease of the magnetic flux within a superconducting metalwhen it is cooled below the critical temperature. That is,superconducting materials reflect magnetic fields.