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Construction and working of Laser Tnpsc


This article is compiled for the topic “ Laser” in General Studies Tnpsc. This note is compiled from Samacheer Kalvi Book.

This note is for revision only. Kindly refer to the Original Samacheer Kalvi book for more clarity.

Laser meaning or laser full form is light amplification by the stimulated emission of radiation. The difference between laser light and ordinary light is the ordinary light emitted from a normal light source is unclear or disordered or technically called Incoherent.

Ordinary light is incoherent because different atom emits different radiation of light at different stages. For the interference of light, coherent (consistent or clear) light is essential. Also, two different sources, cannot give coherent light.

For research needs, two independent coherent light from a single source is obtained. For this, a light technology called LASER is developed to provide highly coherent light.

Fig:1 a) Ordinary Light Source b) Monochromatic Source c) Laser Source

Characteristics of Laser

  • The laser beam of light is monochromatic.

  • The laser beam is clear or coherent.

  • The laser beam is exactly the same phase with each other.

  • The laser beam does not bend or diverge.

  • The laser light is extremely strong and intense.

Working of Laser and principle

Spontaneous and Stimulated Emission

An atom goes through a transition ( change from one state to another) between two energy E1 and E2. It emits or absorbs a photon of some energy E1 – E= hv.

In a system of thermal equilibrium, the atoms in the ground state (N1 ) is greater than the atoms in the excited state (N2). Let the Groud state be E1 and the excited state is E2.

Stimulated Absorption

When Photons having energy E1 – E2  = hv falls on the system the thermal equilibrium, the photon makes the atom in the ground move to the excited state. This phenomenon is called induced or stimulated absorption.

Fig:2 – Stimulated or Induced Absorption

The action that makes the atoms in the ground state to move to the excited state is called pumping.

When Pumping happens due to external energy or force, the atoms in the excited state are increased such that the atoms in the excited state are greater in number than in the ground state. This phenomenon is called Population Inversion.

Fig 3: Population Inversion

The life of atoms that stays excited (excited state) is usually for 10-8 seconds. The exited energy levels with atom lifetime greater than 10-3 seconds and such energy levels are called the Metastable states.

When the excited energy level becomes normal, the atom in the excited state returns to the ground energy state without any external agency. During the transition stage, a photon of energy E1 – E= hv is emitted and is called Spontaneous Emission.

Fig 4: Spontaneous Emission

Fig 5: Stimulated Emission

The atom stays some time at these energy levels if the excited state is metastable (atom lifetime greater than 10-3 seconds).

By using the Photons of energy hv = E2– E1, the atom from the metastable state can be brought down to lower energy levels. A Photon of energy hv = E2– E1 is emitted which brings atoms from the metastable state to lower energy levels. This is called Stimulated Emission or Induced Emission.

“A photon produced by stimulated emission is called secondary photon (or) stimulated photon.”

The secondary photon is usually in phase with the stimulating photon. These photons, in turn, stimulate the emission further and the process continues to produce a chain – reaction which is called Laser action by this reaction, all the emitted photons that have similar energy and frequency are in phase with each other.

Thereby, a highly monochromatic, perfectly coherent, strong radiation is produced in a laser beam.

Condition for Laser Action

The system must have an inverted population, that is fewer atoms in the ground state than the excited state or Excited-state must have more atoms than the ground state. The should be a metastable state.

The emitted photons must stimulate further to create more radiation for this reflecting mirrors are placed at the ends of the system.

Types of Laser

Application of Laser

As the laser is highly monochromatic it is used for a variety of scientific and industrial applications as well as in research.

Industrial Uses

  • It is used for drilling fine holes in diamonds, hard surfaces, hard sheets etc.

  • It is used for cutting thick hard metals.

  • It is used for welding.

  • In the manufacturing of semiconductors, chip, electronic circuits, etc it is used to vapourize the unwanted materials.

  • Also used to determine the quality of the materials

Medical Uses

  • In hospitals, the laser is used to microsurgeries due to its narrow angular spread of the beam.

  • It is used to treat kidney stone, tumour.

  • It is used in Cutting and sealing of blood vessels in brain surgery and retina detachment.

  • It is used in endoscopy.

  • It is used to treat cancer in humans and animals.

Scientific, Engineering and Defence Uses

  • It is used to transmit large messages at a time in radio, television, and telephone as the laser has a single frequency.

  • It is used in optical communication as it uses semiconductor laser is used as the light source.

  • Due to its angular spread, it is used in microwave communication, satellite communication, and rockets.

  • The laser used finding and locking the target in defence.

  • The distance between the earth and other planets, natural satellites can measure using lasers.

  • It is used in the Radar, to accurately determine the distance, speed and direction of the object, and also size and form of the targets.


  • Maser working is almost the same as the working of Laser.

  • When the emitted photon is of microwave frequencies.

  • In Maser, Material used is Paramagnetic ions.

  • The Maser material is usually Chromium, Gadolinium ions doped as impurities in ionic crystals.

  • Another Maser material is Ammonia Gas.

  • Maser is used for analysis in Molecular spectroscopy.

Laser – Tnpsc General Studies Notes Download