Engineering Physics
LASER
LASER is an acronym for Light Amplification by Stimulated
Emission of Radiation. In Laser, the intensity of light is amplified by a
process called Stimulated Emission. Lasers are more powerful radiation
than ordinary light radiation.
Laser has frequency of 1014 Hz to 1015 Hz,due to remarkable
properties of laser it finds major applications in various fields such as
medicine, engineering, fiber optic tele communication, industries, etc.
Characteristics of Laser
The following characteristics, distinguishes a laser beam from an
ordinary light. They are
High degree of coherence. High intensity.
High directionality. High monochromatic.
Laser light is highly powerful and is capable of propagating over
long distances and is not easily absorbed by water.
Let us discuss the four characteristics in detail.
,Laser
High degree of Coherence
The wave trains which are identical in phase and direction are
called coherent waves. Since all the constituent photons of laser beams
possess the same energy, momentum and propagate in same direction with
same phase the laser beam is said to be highly coherent beam.
High Intensity
Due to the coherent nature of laser, it has the ability to focus over a
small area of 10–6 cm2, ie., extremely high concentration of its energy over
a small area, hence called high intensed beam.
High Directionality
An ordinary light source emits light in all possible directions. But,
when laser travels in a parallel beam it can travel over a long distance
without spreading. The angular spread of ordinary light source is 1m/meter,
but the angular spread of a laser beam is 1mm/meter. This reveals the
directionality of the laser beam.
High Monochromatic
The light from a normal source spreads over a range of wavelength,
ie., many wavelengths of light are emitted. But, laser is highly
monochromatic, i.e., it can emit light of single wavelength (or) single
colour.
DIFFERENCE BETWEEN ORDINARY LIGHT AND LASER BEAM
S.No Ordinary Light Laser beam
1. In ordinary light the angular spread is In laser beam the angular spread is
more. (1m/ meter). less. (1mm/m).
2. They are not directional. They are highly directional .
3. It is less intense. It is highly intense.
4. It is not a coherent beam and is not in It is a coherent beam and is in
phase. phase.
5. The radiations are polychromatic. The radiations are monochromatic.
6. Ex: Sunlight, Mercury vapour lamp etc. Ex: He–Ne Laser, CO2 Laser etc.
, Engineering Physics
PRINCIPLE OF SPONTANEOUS AND STIMULATED
EMISSION-EINSTEIN’S A AND B COEFFICIENTS
There are three possible ways through which interaction of radiation
and matter can take place. Among the three types, one is absorption that
is known as Induced Absorption and the other two are emissions.
The emissions of radiation can occur in two ways as suggested by
Einstein. They are Spontaneous Emission and Stimulated Emission.
All the three processes are described by considering an atom having
only two energy levels E1 and E2.
Einstein explained the action of laser beam based on quantum theory
of light.
Absorption
Let the atom be initially in the lower state E1. If a photon of energy
h is incident on the atom in the lower state, the atom absorbs the energy
from incident photon and gets excited to the higher energy state E2. This
process is called Induced Absorption.
R12 = B12N1 ..... (1)
E2
E=E2–E1= h
E1
Fig 2.5 Absorption
The rate of absorption R12 is proportional to the population of the
lower energy level N1 and to the density of incident radiation . Hence,
R12 N1 (or) R12 = B12N1
Where B12 is the proportionality constant which is known as the
probability of absorption of radiation per unit time.
, Laser
Spontaneous Emission
It is a process in which there is an emission of a photon whenever an
atom is transferred from a higher energy state to lower energy state without
the aid of any external force.
R21 (sp) = A21N2 ..... (2)
E2
E=E2–E1= h
E1
Fig 2.6 Spontaneous Emission
The excited atom in the higher energy level E2 spontaneously
returns to the lower energy level E1 with the emission of a photon of energy
h = E2 – E1.
The rate of spontaneous emission of radiation R21 (sp) is proportional
to the population N2 at the higher energy level E2.
i.e., R21 (sp) N2
R21 (sp) = A21N2
Where A21 is the proportionality constant which is known as the
probability of spontaneous emission per unit time.
Stimulated Emission
E2
It is a process in which there is an E= h
emission of photon whenever an atom is
E=E2–E1= h
transferred from a higher energy state to a
lower energy under the influence of an
external force. E1
R21 (st) = B21 N2 ..... (3) Fig 2.7 Stimulated Emission
LASER
LASER is an acronym for Light Amplification by Stimulated
Emission of Radiation. In Laser, the intensity of light is amplified by a
process called Stimulated Emission. Lasers are more powerful radiation
than ordinary light radiation.
Laser has frequency of 1014 Hz to 1015 Hz,due to remarkable
properties of laser it finds major applications in various fields such as
medicine, engineering, fiber optic tele communication, industries, etc.
Characteristics of Laser
The following characteristics, distinguishes a laser beam from an
ordinary light. They are
High degree of coherence. High intensity.
High directionality. High monochromatic.
Laser light is highly powerful and is capable of propagating over
long distances and is not easily absorbed by water.
Let us discuss the four characteristics in detail.
,Laser
High degree of Coherence
The wave trains which are identical in phase and direction are
called coherent waves. Since all the constituent photons of laser beams
possess the same energy, momentum and propagate in same direction with
same phase the laser beam is said to be highly coherent beam.
High Intensity
Due to the coherent nature of laser, it has the ability to focus over a
small area of 10–6 cm2, ie., extremely high concentration of its energy over
a small area, hence called high intensed beam.
High Directionality
An ordinary light source emits light in all possible directions. But,
when laser travels in a parallel beam it can travel over a long distance
without spreading. The angular spread of ordinary light source is 1m/meter,
but the angular spread of a laser beam is 1mm/meter. This reveals the
directionality of the laser beam.
High Monochromatic
The light from a normal source spreads over a range of wavelength,
ie., many wavelengths of light are emitted. But, laser is highly
monochromatic, i.e., it can emit light of single wavelength (or) single
colour.
DIFFERENCE BETWEEN ORDINARY LIGHT AND LASER BEAM
S.No Ordinary Light Laser beam
1. In ordinary light the angular spread is In laser beam the angular spread is
more. (1m/ meter). less. (1mm/m).
2. They are not directional. They are highly directional .
3. It is less intense. It is highly intense.
4. It is not a coherent beam and is not in It is a coherent beam and is in
phase. phase.
5. The radiations are polychromatic. The radiations are monochromatic.
6. Ex: Sunlight, Mercury vapour lamp etc. Ex: He–Ne Laser, CO2 Laser etc.
, Engineering Physics
PRINCIPLE OF SPONTANEOUS AND STIMULATED
EMISSION-EINSTEIN’S A AND B COEFFICIENTS
There are three possible ways through which interaction of radiation
and matter can take place. Among the three types, one is absorption that
is known as Induced Absorption and the other two are emissions.
The emissions of radiation can occur in two ways as suggested by
Einstein. They are Spontaneous Emission and Stimulated Emission.
All the three processes are described by considering an atom having
only two energy levels E1 and E2.
Einstein explained the action of laser beam based on quantum theory
of light.
Absorption
Let the atom be initially in the lower state E1. If a photon of energy
h is incident on the atom in the lower state, the atom absorbs the energy
from incident photon and gets excited to the higher energy state E2. This
process is called Induced Absorption.
R12 = B12N1 ..... (1)
E2
E=E2–E1= h
E1
Fig 2.5 Absorption
The rate of absorption R12 is proportional to the population of the
lower energy level N1 and to the density of incident radiation . Hence,
R12 N1 (or) R12 = B12N1
Where B12 is the proportionality constant which is known as the
probability of absorption of radiation per unit time.
, Laser
Spontaneous Emission
It is a process in which there is an emission of a photon whenever an
atom is transferred from a higher energy state to lower energy state without
the aid of any external force.
R21 (sp) = A21N2 ..... (2)
E2
E=E2–E1= h
E1
Fig 2.6 Spontaneous Emission
The excited atom in the higher energy level E2 spontaneously
returns to the lower energy level E1 with the emission of a photon of energy
h = E2 – E1.
The rate of spontaneous emission of radiation R21 (sp) is proportional
to the population N2 at the higher energy level E2.
i.e., R21 (sp) N2
R21 (sp) = A21N2
Where A21 is the proportionality constant which is known as the
probability of spontaneous emission per unit time.
Stimulated Emission
E2
It is a process in which there is an E= h
emission of photon whenever an atom is
E=E2–E1= h
transferred from a higher energy state to a
lower energy under the influence of an
external force. E1
R21 (st) = B21 N2 ..... (3) Fig 2.7 Stimulated Emission
