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Wave Optics: Wave nature of light, including interference, diffraction, and polarization. Double-slit experiment and Young's modulus. Fresnel and Fraunhofer diffraction. Quantum Mechanics: Wave-particle duality and the uncertainty principle. Schrödinger equation and its solutions. Quantum s...

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  • May 9, 2024
  • 15
  • 2023/2024
  • Class notes
  • Rohinee khandait
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  • physics
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  • engineering

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UNIT IV- LASERS
INTRODUCTION

Laser is one of the greatest discoveries of 20th century. LASER is the acronym, which comes out of the initial
capital letters of the words contained in the description: Light Amplification by Stimulated Emission of
Radiation. The name itself specifies that, the light is amplified by the process of stimulated emission, which was
discovered by Einstein in 1920. But the first successful solid - state laser was invented in 1960, i.e., it almost took
40 years to put the known theoretical concepts into a practical. Laser is an artificial light source, which is very
much different from many of the traditional light sources. Unlike ordinary light, laser produces highly coherent,
intense, monochromatic, polarized light. Because of these unique properties, laser has wide variety of
technological applications ranging from welding, medicine, communication, defense, compact disc players, digital
video disc players, bar code readers, laser printers, eye surgery equipment, optical communication networks and
as a tool in many areas of scientific research, thereby acquiring a status of indispensable part our daily life.
Different types of lasers are commercially available, ranging in size from devices that can rest on a fingertip to
those that fill large buildings.
 Laser is a quantum electronic process which produces highly coherent, intense, monochromatic, polarized
light.
 Laser is a process that uses the principle of amplification of electromagnetic waves due to stimulated
emission of radiation and operates in the Infrared (IR), Visible (VIS) or Ultra -violet (UV) region.
WORKING PRINCIPLE OF LASER
LASER works on the principle of quantum theory of radiation. When an atom absorbs energy (hν), it goes to an
orbit of higher energy level (E2), when it comes back to the stable state (E1), it emits radiation in the form of
photon having energy (hν).
hν = E2 – E1
where, h is planks constant and ν is the frequency of photons.

CHARACTERISTCS OF LASER
Laser has wide applications due to its important characteristics.
1) Directionality
Light from traditional or ordinary sources of light like sun, electric bulb, sodium vapor lamp etc. is emitted
uniformly in all directions. It is non-directional. In Lasers, the stimulated photons travel in same direction parallel
to the cavity axis. Hence laser beam is a highly directional beam of light.
2) Negligible divergence
Light from ordinary source of light travel in form of spherical wavefronts and is highly divergent. Light from laser
source travel in form of plane wavefronts and form a bundle of parallel rays. Thus, laser light has almost
negligible divergence.
3) High Intensity
For ordinary source of light, intensity of light goes on decreasing with distance. The narrow beam of laser light
has its energy concentrated in a very small area of space. Hence its intensity is high and remains constant over
large distances. A 0.1mW laser has intensity which is 10,000 times more intense than sunlight falling on earth’s
surface.
4) High degree of Coherence
The light from ordinary sources diverges and contains light waves of random phases. In case of laser source, all
the stimulated photons are in same phase with respect to each other. Thus Laser has a high degree of coherence
with coherence length of few kms.
5) High Monochromaticity
The light from ordinary source of light is not monochromatic. It contains random wavelengths. It spreads over a
wavelength range of 100 to 1000Å.For laser as a source of light, all the stimulated photons have same wavelength
and frequency. The light emitted is highly monochromatic with spread over a wavelength range of less than 10Å.
6) Polarization
Laser light is highly polarized, while ordinary light is unpolarized.

, Applications:
Engineering (Industrial) Applications:
Welding – Welding is the joining of two or more pieces into a single unit. Main advantage of laser welding is a
contact less process and hence no possibility of impurities into the joint.
Drilling – In drilling, the laser beam is focused on small area and material get vaporized and creates a hole. With
laser it is possible to drill very small hole and in desired direction.
Cutting – A wide range of materials can be cut by laser. The material includes paper, wood, cloth, glass, quartz,
ceramics, steel, etc. the advantage of laser cutting is that it is fine and precise.
Hardening- It is used to give heat treatment (hardening process) in automotive industries.
B] Medical applications:
Bloodless cancer surgery –Bloodless cancer surgery can be performed with laser. As laser beam can be focused
on a infected area and harmful tissues can be destroyed without damaging surrounding region.
Eye surgery – Laser can be used for the spot welding of detached retina in the eyeball.
Plastic Surgery – Plastic surgery involve replacement of skin from some part of body and grafting it at some
other part. For this laser of yellow colour is used.
C] Communication:
Laser light is used for optical communication. It is suitable for long distance communication.
D] Electronic Industries:
Bar-code Reader – A low power (He-Ne) Laser beam is used to read the bar code, which contains the
information regarding the item and its manufacture.
Compact Disc –The laser light is used in CD audio player, CD-ROM drives, Floppy disks, memory cards and
also in optical reading.
Electronic circuits – It is used in manufacturing of electronic circuits and removal of faulty components in
circuits.
Apart from above applications. Lasers are used in holography, astronomy, in LIDAR (Light detection and
ranging), It is used in weapon guidance in wars in laser guided missiles. It is used in entertainment field too. It is
used in laser printer, xerox, copier and facsimile machines too. It is used in scientific research.

Quantum transitions (interaction of radiation with matter)

Transition of an atom from one energy level to another is called Quantum Transition. Generally, quantum
transition occurs when atom interacts with radiation. There are three types of quantum transitions
1. Absorption 2. Spontaneous emission 3.Stimulated emission
I) Absorption
When an atom in ground state E1 absorbs the incident photon of energy hν=E2-E1 and jumps to upper
excited state E2 then such a transition is called Stimulated absorption or absorption. The process can be
represented by equation A + hν →A*and is shown in figure1.




Figure1: Absorption process
The number of absorption transitions occurring in the material (Nabs.) in time interval ∆t is given by,
N abs. = B12 N1 Q ∆t
where N1= number of atoms in the ground state,
Q = photon density of the incident beam and
B12 = Einstein’s coefficient for stimulated emission.

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