LASER

Introduction to LASER: - Laser is the acronym for light amplification stimulated emission of radiation. The first successful laser was Ruby laser, invented in 1960 by American physicist Theodore Maiman. It is use to provide intense, monochromatic & completely coherent light. It works on the principle of stimulated emission.

 Properties of a laser:-

1) Monochromatic: - Single color & single wavelength.

2) Divergence & Directivity: - Beam is well collimated (Parallel) & travels long distance with very less spread.

3) Coherence: - All the individual waves of light moves - precisely (Exactly) together through time & space.

4) Intensity: - The intensity of the laser beam is very high. E.g. If a person is allowed to observe an ordinary light by 100 W bulb, he receive only 1/1000th of it at a distance of 1 foot. But he observes a laser beam at the same distance, the laser beam penetrate through eyes & it will damage eyes.

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Principle of Laser 

To understand working principle of laser, consider a medium consisting of atoms being excited from the lower energy level (Ea) to upper energy level (Eb), by absorption of photon. Let the population of Ea is Na & population of Eb is Nb.

Let the atoms be in thermal equilibrium i.e.; no photons generated or lost. However, when atoms are subjected to an external light of frequency the following three process occurs-

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(1) Absorption - An electron living in the lower energy level Ea, may absorb the incident photon & Jump to the upper energy level Eb. This transition is known as absorption.

The rate of absorption will be proportional to population in the lower energy level & density of photon.

(2) Spontaneous Emission - An excited electron can stay at upper energy level for an average time. After this average time, the excited electron comes to lower energy level on its own. During the transition, energy releases in the form of photon. This process is known as spontaneous emission.

The number of photons generated will be proportional to the population of the excited level.

Features –

a) This process is uncontrollable.

b) The emitted photon can move in any random direction.

c) Since the photon moves in all direction the light is neither monochromatic nor coherent. And the intensity of light goes on decreasing.

(3) Stimulated Emission - If the photon, interact with the excited atom whose electron is at most upper energy level then transition takes place & electron comes lower energy level. During the transition another photon is generated. Hence there is two photon one is original & the other is generated, but the both is completely coherent. This process is known as stimulated emission.

The rate of stimulated emission is directly proportional to population in the upper energy level (Nb) & density of original photon.

Features –

a) This process is controllable.

b) All photon moves in same direction.

c) Since the all photon moves in the same direction, the light is monochromatic & coherent.