Laser
The term LASER stands for light amplification by stimulated emission of radiation.
The laser is a device which amplifies light waves.
The area of laser and applications as been expanding in the various fields with tremendous speed.
Characteristics of laser
Laser radiation is different from normal light radiation. The characteristics of laser are given below.
1) Laser radiation is monochromatic. It contains only one particular wavelength of light.
2) Laser radiation is coherent. In emitted radiation, all the photons are having the same phase.
3) Laser radiation is highly directional. Laser emits a tight beam of light so it travels a long distance.
4) Laser radiation is very intense. The intensity and hence the brightness of the radiation is high.
Principle of laser
Laser works on the principle of quantum theory of radiation.
when an atom in the lower energy level absorbs the photon energy of incident radiation, it excited to the higher energy level.
when it jump back to the lower energy level, the energy difference is emitted in the form of radiation.
Spantaneous emission
spantaneous emission is the process of photon emission takes place immediately without any inducement during the transition of atoms from higher energy levels to lower energy levels.
It is a random process in which the photons are emitted at different time.
so that in the emitted radiation each photon has different phase and are incoherent.
The number of spantaneous transition taking place per unit volume is directly proportional to the number of atoms in the higher energy level.
Stimulated emission
Einstein found that a new process called stimulated emission to increase the number of transition of atoms from higher energy levels to lower energy levels.
stimulated emission is the process of photon emissions takes place by an inducement given by another photon incident on the atoms in higher energy levels.
During this process, the stimulating photon and the photon emitted by the excited atom arre emitted simultaneously in the same direction. Also both energies are equal.
Hence they are identical in phase, directional in phase, direction and frequency and are coherent. This process of stimulated emission is used to produce laser beam.
The number of stimulated transition taking place per unit time per unit volume is directly propostional to the number of atoms in the higher energy level and the number of photons in the incident radiation.
when a single photon hits an atom in the higher energy level, two photons are emitted by stimulated emission.
Then these two photons hit on two atom of higher level, four photons are emitted. This process is continued as a chain reaction and the photons are getting multiplied.
Finally it leads to emit the powerful beam of laser light. This is called light amplification by stimulated emission of radiation.
This amplification takes place only if there are more number of atoms in the excited state than in the ground state.
Population inversion
Population inversion is the established of the situation to make more number of atoms in the excited state required for the chain reaction of stimulated.
For achieving the population inversion, the number of atoms must be raised continuously in the excited state. when a sufficient energy is given to the atoms in the ground state , they excited to the higher energy levels.
this process is called pumping. if the atoms in the ground state are bring to the excited state by means of light energy, the process is called optical pumping.
Types of laser
The important types of laser are solid state lasers. Gas lasers liquid laser, dye lasers and semi conductor laser.
Solid state lasers : Ruby laser, Nd - YAG laser
Gas lasers : He-Ne laser, Argon iron laser
Liquid laser : organic chelate laser
Dye laser : Rhodmine 6 G laser
Semi conductor laser : Ga- As diode laser
Ruby laser
The ruby laser consists of single ruby rod surrounded by a flash tube in the form of spiral. one end of the ruby rod is silvered and the other end is semi silvered and semi transparent.
The ruby rod consist of a crystal of aluminium atoms are replaced by chromium atoms. The chromium atoms have the property of absorbing green light.
When the external green light is flashed by the flash tube, the electrons in the chromium atoms gained energy and go to the higher energy levels.
In these levels they are highly unstable then they stimulated and go back to the stable orbit by the emission of extra energy in the form of radiations.
These stimulated emission of radiations undergo multi reflection within the ruby rod with the help of mirrors at the both ends.
As the result result a powerful coherent and monochromatic beam of red radiation emerges out through the semi transparent mirror at one end. These radiation have high intensity and can travel over long distances without loss of intensity.
Uses of laser
laser has a number of practical uses. They are
1) Laser is used to find the long distances by direct pulse reflection method.
2) It is used in the conversion of light into electric current.
3) It is used to produce high temperature to melt and the metals in a very short period.
4) It is used to bore holes in a hard substances like tungsten and diamond.
5) It can be modulated to transmit hundreds of messages at a time on radio, television and telephone.
6) It can accommodate 80 million television channels.
7) The big size objects like Aeroplane s, missiles can be destroyed in a few seconds by passing a powerful laser beam on them. For this reason laser is called a " death ray " instrument.
8) In surgery a laser beam is used in spot welding of a detached retina in the eye ball. It is also used for painless drilling and welding the surface of a tooth to prevent its decay.
9) A suitable telescope can send the laser beam to the moon to obtain a sharper definition of the moon's surface.
10) The laser beam is not easily absorbed by water. So it can be used in under water communication between submarines.
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