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The property of emission of light by solids due to electron transitions. The phenomenon exhibited by some semiconductors, particularly compound semiconductors with direct band gaps in which electrons (carriers) are excited into higher impurity levels from which they fall to equilibrium states, giving off light.

Longitudinal wave motion
Wave motion in which particles of medium vibrate along the direction of propagation of wave.

LINEAC(Linear Accelrator)
Device/ arrangement to accelerate charged particles like electrons/protons to high energies of order of MeV. It is an arrangement of lengthy volume containing multiple tubes called drift tubes whose potential is varied by predetermined RF oscillator such that the charged particle leaving each tube will leave with high energy.

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.

Lienard Wiechart Potential
It is magnetic vector which is used to find radiation from accelerated charges moving on specified trajectory path.

Lorentz-Fitzerald contraction
Every rigid body appears to have maximum dimensions when at rest relative to the observer. Its dimensions appear to be contracted in the direction of relative motion by factor sqrt(1-v2/c2) when it moves with velocity ‘V’ relative to observer.

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.

Laws of refraction
i) Incident point, refracted point and normal drawn to plane of refraction are coplanar.
ii) The relationship between angles of incidence () and refraction is expressed by

Lissajous figures
When a particle is influenced simultaneously by two simple harmonic motions at right angles to each other, the resultant motion of particle traces a curve called Lissajous figure. Lissajous figures are helpful in determining ratio of time periods of two vibrations.

Lattice parameters
The unit cell geometry is completely defined in terms of six parameters; the three edge lengths and three inter axial angles which we referred to as lattice parameters.

It is acronym for Light Amplification by Stimulated Emission of Radiation. Laser is that device in which an intense, monochromatic, coherent and unidirectional light beam is obtained.

Law of Conservation of energy
Energy can neither be created nor destroyed, but it can be transformed from one form to other.

Law of static friction
It states that static friction is independent of area of contact, depends on nature of surfaces in contact & maximum value is directly proportional to normal force.

Law of kinetic friction
Kinetic friction is independent of area of contact; kinetic friction is independent of relative speed between two contact surfaces and is directly proportional to normal force.

Law of Conservation of angular momentum
When no external torque acts, then angular momentum of rotating system remains conserved.

Longitudinal Stress
When a force is applied on a body such that there is a change in the length of the body, the longitudinal force per unit area is called longitudinal stress.

Longitudinal strain
It is the change in length per unit original length or it is fractional change in length.

London penetration depth 
It is the distance inside the surface of superconductor at which flux density falls to 1/e of its value at the surface.

Lyman series
The spectrum of wavelength falling in Ultravoilet region of electromagnetic spectrum due to transition of electrons from higher energy states (orbits) to first orbit (inner most orbit) is called Lyman series.

Long range alpha particles
If the parent alpha emitter emits alpha particles when it is in an excited state then we get long range alpha particles.

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