S6: Physics

Key unit Competence: Analyze the applications of LASER.
My goals

INTRODUCTORY ACTIVITY

A man has tied all forms of advancements from traditional methods of solving
problems to advanced methods by use of different technologies. Among
other technological advancements, discovery of Laser that is a part of visible
light under electromagnetic waves has had a great impact in solving many of
our problems.
10.1 CONCEPT OF LASER
ACTIVITY 10.1
The acronym LASER stands for Light Amplifier by Stimulated Emission of
Radiation. This expression means that the light is formed by stimulating a
material’s electrons to give out the laser light or radiation.

The laser is a device that uses the ability of some substances to absorb
electromagnetic energy and re-radiate it, as a highly focused beam of
monochromatic and synchronized wavelength radiation. In 1953 Charles H.
Townes, with graduate students James P. and Herbert J., produced the first
Microwave Amplifier by Stimulated Emission of Radiation (MASER), 
as a device operating in the same way as a laser, but amplifying microwave
radiations.

This system could release stimulated emissions without falling to the ground
state, and thus maintaining a population inversion. A laser is a device that
emits light through a process of optical amplification based on the stimulated
emission of electromagnetic radiation. That is, the laser is a light source that
produces a beam of highly coherent and very nearly monochromatic light

because of cooperative emission from many atoms. 

10.1.1 Absorption, Spontaneous emission and Stimulated emission
ACTIVITY 10.2
1. Using scientific explanations, Explain the meaning of the following
terms
I. Absorption
II. Stimulated emission
III. Spontaneous emission
2. Electrons can jump from excited to ground state; does it absorb or
radiate energy. Explain your reasoning.
3. Write an equation that would be used to calculate the energy radiated
by an electron when it jumps from one energy level to another. Explain
each term used in the equation.

4. What do you understand by the term population inversion?

a. Absorption
During the process of absorption, a photon from the source is destroyed and

the atom which was at the ground state is promoted to the excited state. 

In normal cases the excited states are less populated than the ground state.

b. Spontaneous emission.

An atom or an electron can move from one energy level to another. A photon is
released when an electron moves from a higher energy level to a lower energy
level. The release of photon (a particle of light) is called spontaneous emission.

At the excited state, an atom will drop to a lower level by emitting a photon
of radiation in a process called spontaneous emission. It emits the photon
spontaneously after an average time τ called the spontaneous lifetime of the
level. This time depends on the atomic species; some levels have long lifetime
measured in seconds, whereas others are relatively short on the order of
nanoseconds or less. This lifetime determines the ability of the emitting atom

to store energy and will affect the efficiency of sources.

frequency as the atomic frequency, there is a finite probability that this wave
will force the atom to undergo the transition E2 → E1 .

In this case the energy difference between the two levels is emitted in the form
of electromagnetic wave that adds to the incident one. This is the phenomenon
of stimulated emission. There is a fundamental difference between the
spontaneous and stimulated emission processes because in spontaneous
emission one photon is emitted and in stimulated emission both incident and
emitted photons are observed. 

10.1.2 Laser principle

The principle of operation remains the same though there is a wide range of
lasers. Laser action occurs in three stages: photon absorption, spontaneous
emission, and stimulated emission. The particle of the material, which undergoes
the process of excitation, might be an atom, molecule, or ion depending on the
laser material. This principle is based on the principle of stimulated emission of
radiation, the theory that was discussed by Einstein in 1917.The whole concept
was discussed in the previous section.

The photon emitted during stimulated emission has the same energy as the
incident photon and it is emitted in the same direction as the latter, thus, getting
two coherent photons. If these two coherent photons are incident on other
two atoms in E2, then it will result in emission of two more photons and hence
four coherent photons of the same energy are emitted. The process continues

leading to doubling of the present number of photons.

If the process is made to go on chain, we ultimately can increase the intensity
of coherent radiation enormously. In figure above, such amplification of the
number of the coherent photons due to stimulated emission is shown.

The necessary condition for this type of amplification of light intensity by
stimulated emission of radiation is that number of atoms in the upper energy
state E2 must be sufficiently increased.

10.1.3 Population inversion

Population inversion: This is the process of increasing excited electrons in
higher energy levels. This is the redistribution of atomic energy levels that

takes place in a system so that laser action can occur.

There are different methods of achieving population inversion in atomic states
that is essential requirement to produce laser beam.

Normally, most of the atoms in a medium are in the ground state of energy E0
.There are four different methods of making these atoms to excited states.


i. Excitation with the help of photons. If the atoms are exposed to an
electromagnetic radiation of high frequency, then there is selective
absorption of energy and thus atoms are raised to excited state.
ii. Excitation by electrons. This method is used in some gas lasers. Electrons
are released from the atoms due to high voltage electric discharge
through a gas. These electrons are then accelerated to high velocities
due to high electric field inside a discharge tube. When they collide with

neutral gas atoms, a fraction of these atoms are raised to excited state 
e + X → X ^∗+ e  Where X is an atom in ground state and ∗ X is an atom in
excited state
iii. Inelastic collision between atoms. If a gas contains two different two
different kinds of atoms X and Y, then during electric discharge through
the gas some of the atoms are raised to excited state.


iv. Excitation by chemical energy. Sometimes, an atom or a molecule can be a
product of a chemical reaction and can be produced in its excited state. An
example is hydrogen combining with fluorine to form hydrogen fluoride

HF that is in excited state.

10.1.4 Laser structure
ACTIVITY 10.3

1. From what you know about LASER, what could be the components
of laser
2. Are all parts on laser Light Similar? Explain your reasoning.

In general case laser system consists of three important parts: Active medium
or amplifying medium, the energy source referred to as the pump or pump

source and the optical resonator consisting of mirrors or system of mirrors.

Pumping Mechanism.

Pumping is the process of supplying energy to the laser medium to excite to
the upper energy levels. To have this mechanism, it depends on the existence of
interactions between light from pump source to constituents of active medium.
Usually, pump sources can be: electrical discharges, flash lamps, arc lamps,
light from another laser, chemical reactions and even explosive devices. Most
common lasers use electrical or optical pumping. The type of pump source used

depends essentially on the gain medium.

Active Medium

The active medium is the major determining factor of the wavelength of
operation, and other properties of the laser. The gain medium is excited by the
pump source to produce a population inversion, and it is where the spontaneous
and stimulated emission of photons take place, leading to the phenomenon of
optical gain or amplification. The gain medium may be a solid crystal like a
ruby, a liquid dye, gases like CO₂ or He-Ne or semiconductors. The gain medium
for some lasers like gas lasers is closed by a window under the Brewster’s angle

to allow the beam to leave the laser tube. 

Optical resonator or Optical cavity

The optical resonator or optical cavity is a system of two parallel mirrors placed
around the gain medium that provide reflection of the light beam. Light from
the medium produced by the spontaneous emission is reflected by the mirrors 
back into the medium where it may be amplified by the stimulated emission.
Mirrors are required for most lasers to increase the circulating power within
the cavity to the point where gains exceed losses, and to increase the rate of
stimulated emission. One of the mirrors reflects essentially 100% of the light,
while the other less than 100% and transmits the remainder. Mirrors can be
plane, spherical or a combination of both. Here represented are the common

cavities configuration that can be used:

10.1.5 Checking my progress

1. What do you understand by the term LASER?
2. Write in full the acronym L.A.S.E.R
3. In your own words, explain how laser light is produced.
4. Explain the meaning of population inversion and discuss how an atom
can be put into excited state.
5. What is the energy of the laser light that propagates with a frequency
of 1010 Hz in gaseous medium. (Given that the plank’s constant

6. What are the three major components of laser?

7. Using diagrams, explain all the types of optical cavity.

10.2 PROPERTIES OF LASER LIGHT
ACTIVITY 10.4

a. Using the ideas about electromagnetic radiations, what are
characteristics of laser light?
b. Do you think all different kinds of laser light have the same
properties? Give reasons to support your answer.

The laser light is not like any other light emitted by usual sources found in
nature. This special light emitted by the laser, has three properties according
to its usefulness in many applications: Coherence, Monochromaticity and

Collimation or Directionality.

10.2.1 Coherence

Coherence is the most interesting property of laser light. All photons
emitted, are exactly in the same phase, they are all crest and valley at the
same time. It is brought about by the mechanism of the laser itself in which
photons are essentially copied.The good temporal coherence is essentially for
Interferometry like in Holography. Coherence is not trivial and is brought about

by the amplification mechanism of the laser.

10.2.2 Monochromaticity

Monochromaticity is the ability of the laser to produce light that is at one
wavelength λ. It is a requirement for coherence since photons of different
wavelengths cannot be coherent. When white light is dispersed through a
prism, you note that it is composed of an infinite number of wavelengths of
light covering the entire visible spectrum as well as into the UV and IR regions.
However, no light source is perfectly monochromatic. Lasers tend to be relatively 
monochromatic and this depends on the type of laser. Monochromatic output,
or high frequency stability, is of great importance for lasers being used in

Interferometry.

10.2.3 Collimation or Directionality

Collimation or directionality is the property of laser light that allows it to stay in
one direction at the strait line, confined beam for large distances. This property
makes it possible to use the laser as a level in construction or to pinpoint
speeders on a highway. This highly directional laser light is determined by the

mechanism of the laser itself.

10.2.4 Checking my progress

1. Choose the correct group of terms that are properties of laser light.

a. Coherent, unpolarized, monochromatic, high divergence

b. Monochromatic, low divergence, polarized, coherent

c. Polychromatic, diffuse, coherent, focused

d. Monochromatic, birefringent, nonpolarized, coherent

2. Which of the following properties of laser light enables us to use it to

measure distances with great precision?

a. All the light waves emitted by the laser have the same direction

b. The light waves are coherent

c. The light waves are monochromatic

d. The individual waves effectively work like a single wave with very

large amplitude.

3. Explain how coherence, monochromatic and collimation are

interconnected.

4. All light in laser light are produced and found to be in the same phase.

How does this help in the formation of 3D images?

5. Laser light can be used as a level. Which special feature that makes it be

used

10.2.4 Checking my progress

1. Choose the correct group of terms that are properties of laser light.
a. Coherent, unpolarized, monochromatic, high divergence
b. Monochromatic, low divergence, polarized, coherent
c. Polychromatic, diffuse, coherent, focused
d. Monochromatic, birefringent, nonpolarized, coherent
2. Which of the following properties of laser light enables us to use it to
measure distances with great precision?
a. All the light waves emitted by the laser have the same direction
b. The light waves are coherent
c. The light waves are monochromatic
d. The individual waves effectively work like a single wave with very
large amplitude.
3. Explain how coherence, monochromatic and collimation are
interconnected.
4. All light in laser light are produced and found to be in the same phase.
How does this help in the formation of 3D images?
5. Laser light can be used as a level. Which special feature that makes it be
used

10.3 APPLICATIONS AND DANGERS OF MISUSE OF LASER
10.3.1 Applications of lasers.

ACTIVITY 10.5

a. Having studied LASERS, where do you think in real life LASERS
are helpful?
b. From your experience, have you ever used LASER light?
c. Other than using it by yourself, what are other places where laser

light is applied

There are many interesting uses for lasers, depending on the special characteristic
being applied. Laser Diodes are used in a wide range of applications. Partial
lists of those applications include:
i. They are used in common consumer devices such as DVD players, bar code
scanners; CD ROM drivers; laser disc and other optical storage drivers;
laser printers and laser fax machines; sighting and alignment scopes;
measurement equipment; free space communication systems; pump
source for other lasers; high performance imagers; and typesetters. CD
players have lasers. Light from the laser in CD player reflects off patterns
on CD’s surface. The reflected light is converted to a sound wave.

ii. Laser beams can be used in diverse fields of science and technology. Like
in the control of motion of moving objects like aircrafts or missiles. This
method thus makes it possible for a missile to hit a certain target.
iii. Because of high directional property, lasers are used to measure distances
accurately. A laser beam is sent and the time taken for it to be reflected
back is measured. Using this idea, the distance can thus be measured.
iv. Because laser beam can be focused into a small spot, it can thus be used
to cut minute holes onto a material.
v. The very high intensity of laser beam means that the amplitude of the 
corresponding electromagnetic wave is very large. So it is possible to
investigate the non linear optical properties of different materials with
the help of laser light.
vi. Lasers are also used in industry for cutting materials such as metal and
cloths. and welding materials
vii. Doctors use lasers for surgery and various skin treatments
viii. They are used in military and law enforcement devices for marking targets
and measuring range and speed.
ix. Laser lighting displays use laser light as an entertainment medium.
x. Lasers also have many important applications in scientific research .
In a tabular way, we can have a summary of different types of lasers and their
applications.

The following are types of lasers and their Applications

a. Gas Lasers:


b. Solid State Lasers:








10.3.2 Dangers of lasers
ACTIVITY 10.6
Laser light is used in many areas like industries, offices, airports and
many other places. Do you think long exposure of laser light is harmful?
1. Why do you think so?
2. What makes these lasers harmful if mis-used? Give a scientific

reasoning

You should be careful when dealing with lasers, because they can have a negative
impact when exposed to your body. Among other negative effects, some of them
are discussed below .
i. If directly exposed to our skin, it burns the skin
ii. When absorbed by skin, Laser light reacts with body cells causing cancer.
iii. Because of their high energy, it affects eyes if exposed to them
iv. Lasers can affect cells of a human being. This leads to mutation

Because of the negative effects of lasers, care must be taken to avoid all the

risks of being affected by lasers.

10.3.3 Precaution measures

ACTIVITY 11.7

a. Observe the picture above clearly. Using scientific reasoning
explain why the people performing the activity above are putting
on protective wear as shown.

b. Building on what you have discussed in a) above, what precautional
measures can you take to avoid negative effects of LASERS if at all

you were working in a place exposed to them.

The following are some of the measures one can take to avoid the negative
effects of lasers.

i. For any one working in places where there are incidences of being exposed
to laser light, one should wear protective clothes, glasses and shoes so
that there is no direct exposure of these radiations on to the body.
ii. One should minimize the time of working with lasers.
iii. Areas that are exposed to these radiations should be warning signs and
labels so that one can be aware of places/areas where laser light is used.
iv. Safe measures like Use of remote control should be used to avoid direct
exposure of these radiations (LASER light).
v. People should be given trainings on how to handle lasers.

vi. There should also access restrictions to laboratories that use laser

10.3.4 Checking my progress
1. Discuss all the negative effects of laser light.
2. Using vivid examples, explain how one can prevent him or herself of all
dangers caused by laser light.
3. We have seen that laser light is good and at the same time bad. Using
your personal judgement, which side outweighs the other. Give scientific
reasons.
4. Depending on your judgement in (3) do you think man should continue

using laser light?

END UNIT ASSESSMENT 1
A. Multiple choice
Copy the questions below to your exercise and chose the best alternative
that answers the question.
1. What does the acronym LASER stand for?
a. Light Absorption by Stimulated Emission of Radiation
b. Light Amplification by Stimulated Emission of Radiation
c. Light Alteration by Stimulated Emission of Radiation
2. The acronym MASER stands for?
a. Microwave Amplification by Stimulated Emission of Radiation
b. Molecular Absorption by Stimulated Emission of Radiation
c. Molecular Alteration by Stimulated Emission of Radiation
d. Microwave amplification by Stimulated Emission of Radio
waves
3. What is one way to describe a Photon?
a. Solid as a rock
b. A wave packet
c. A torpedo
d. Electromagnetic wave of zero energy
4. Which of the following determines the color of light?
a. Its intensity
b. Its wavelength
c. Its source
d. Some information missing
5. Among the three examples of laser listed below, which one is
considered “eye safe”?
a. Laser bar-code scanners
b. The excimer laser
c. Communications lasers
d. YAG
6. Why are lasers used in fiber optic communications systems
a. The government has mandated it
b. They can be pulsed with high speed data
c. They are very inexpensive

d. They are not harmful

7. Lasers are used in CDs and DVDs. What type of laser is used in these
players?
a. Semiconductor
b. YAG
c. Alexandrite

d. All the above

8. The best reason why lasers used in “Laser Printers” is
a. They can be focused down to very small spot sizes for high
resolution
b. They are cheap
c. They are impossible to damage
d. They are locally available
9. As wavelength gets longer, the laser light can be focused to…
a. Larger spot sizes
b. Smaller spot sizes
c. Large and small spot sizes
d. None of the above
10. Among the following, which color of laser has the shortest wavelength?

a. Yellow                                                 c. Blue

b. Red                                                     d. Green

11. What property of laser light is used to measure strain in roadways?

a. Intensity

b. Power

c. Coherence

d. All the above

12. What is the type of laser used most widely in industrial materials

processing applications?

a. Dye Laser                                   c. YAG laser

b. Ruby Laser                                d. Carbon Dioxide Laser

13. Why are lasers used for cutting materials

a. It never gets dull               d. It has a small “heat affected zone”

b. Accuracy                              e. Smoother cuts

c. Repeatability                      f. All of the above

14. The Excimer laser produces light with what wavelength?

a. Visible

b. Ultraviolet

c. Infrared

d. All the above.

15. Most lasers are electrically inefficient devices.

a. True

b. False

16. Chemical lasers use………………. to produce their beams.
a. Excessive amounts of electrical power
b. Small amounts of electrical power
c. No electrical power
d. Other lasers
17. What type of laser could cause skin cancer if not used properly?
a. Red semiconductor laser c. Blue semiconductor
b. Excimer laser d. YAG laser.


B. Structured questions

18.
a. What do you understand by term LASER?
b. Depending on the nature and what laser is made of, Laser is
classified into different types. Discuss at least 5 types of lasers.
19 The following are basic characteristics of laser light. With clear
explanation, what does each imply as connected to laser light.
a. Coherence
b. Monochromaticity
c. Collimation
20. a. With the aid of diagram Explain the meaning of the following terms
I. Spontaneous Absorption of light
II. Stimulated Emission cause harm if mis-used In what ways is
laser light harmful.
III. Spontaneous Emission
IV. Population inversion
b. Laser light have been employed in different areas. This has helped
man in solving different problems. What are some of the areas
where laser light is employed.
c. Though laser light is very important in different activities, it can

also