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Laser is an acronym for light amplification by stimulated emission of radiation. Laser is a device for producing a highly intense narrow beam of nearly monochromatic light. In 1917, Einstein introduced the concept of stimulated emission of radiation which led to the invention of laser.
When an electron from an orbit of higher energy (E2) jumps to an orbit of lower energy (E1), a photon of energy = (E2  E1) = hγ is emitted, where γ is the frequency of the photon emitted. This is called spontaneous emission.
An electron in the ground state of an atom can be raised to a higher energy state by shining a photon of suitable energy on it. This process is called excitation.
An electron in the excited state may remain for 108 s in that state. If another photon of energy hγ = (E2  E1) is incident on it, then the electron in the excited state is made to jump to the lower energy state emitting an additional photon of same frequency. This is called stimulated emission. This makes the laser action possible.
Characteristics of Laser
  1. Laser light is highly monochromatic.
  2. It is highly directional.
  3. It is coherent.
  4. It has higher intensity than ordinary light.
  1. Laser ranging is the technique used to calculate the distance between two objects using laser reflectors.
  2. To study the molecular structure of the material in laser Raman spectroscopy.
  3. Used in optical surgery, treatment of dental decay and skin diseases.
  4. Used in optical fibre communication.
  5. Used in holography.

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