LASER

TOPIC: -                                                                LASER

INSTRUCTIONAL MATERIALS: - Ray –box, torch                light

REFERENCES BOOK: -

FARINDE O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.

M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY SCHOOLS, Africana first publishers.

INTERNET   

PREVIOUS KNOWLEDGE: - Students have been familiar with gadgets uses electromagnetic principles.

 OBJECTIVES: -  At the end of the lesson students should be able to: -

1.       State the meaning of the acronyms LASER.

2.       List the principles of working of a LASER.

3.       Mention characteristics of LASER

4.       State the application of LASER in medicine and industry.

CONTENT: -

LASER

The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser is a device that amplifies or increases the intensity of light and produces highly directional light.

Laser not only amplifies or increases the intensity of light but also generates the light. Laser emits light through a process called stimulated emission of radiation which amplifies or increases the intensity of light. Some lasers generate visible light but others generate ultraviolet or infrared rays which are invisible.

Laser light is different from the conventional light. Laser light has extra-ordinary properties which are not present in the ordinary light sources like sun and incandescent lamp.

The conventional light sources such as electric bulb or tube light does not emit highly directional and coherent light whereas lasers produce highly directional, monochromatic, coherent and polarized light beam.

In conventional light sources, excited electrons emit light at different times and in different directions so there is no phase relation between the emitted photons.

On the other hand, the photons emitted by the electrons of laser are in same phase and move in the same direction. Einstein gave the theoretical basis for the development of laser in 1917, when he predicted the possibility of stimulated emission. In 1954, C.H. Townes and his co-workers put Einstein’s prediction for practical realization.

They developed a microwave amplifier based on stimulated emission of radiation. It was called as MASER (Microwave Amplification by Stimulated Emission of Radiation. Maser operates on principles similar to laser but generates microwaves rather than light radiation.

In 1958, C.H. Townes and A. Schawlow extended the principle of masers to light. In 1960, T.H. Maiman built the first laser device.

Principles of working of a laser

In lasers, photons are interacted in three ways with the atoms:

1.       Absorption of radiation                                 2. Spontaneous emission                             3. Stimulated emission

Absorption of radiation is the process by which electrons in the ground state absorbs energy from photons to jump into the higher energy level.

The electrons orbiting very close to the nucleus are at the lower energy level or lower energy state whereas the electrons orbiting farther away from the nucleus are at the higher energy level. The electrons in the lower energy level need some extra energy to jump into the higher energy level. This extra energy is provided from various energy sources such as heat, electric field, or light.

Unlike the spontaneous emission, the stimulated emission is not a natural process it is an artificial process.

In spontaneous emission, the electrons in the excited state will remain there until its lifetime is over. After completing their lifetime, they return to the ground state by releasing energy in the form of light.

In stimulated emission, the electrons in the excited state need not wait for completion of their lifetime. An alternative technique is used to forcefully return the excited electron to ground state before completion of their lifetime. This technique is known as the stimulated emission. 

When incident photon interacts with the excited electron, it forces the excited electron to return to the ground state. This excited electron release energy in the form of light while falling to the ground state.

In stimulated emission, two photons are emitted (one additional photon is emitted), one is due to the incident photon and another one is due to the energy release of excited electron. Thus, two photons are emitted.

The stimulated emission process is very fast compared to the spontaneous emission process.

All the emitted photons in stimulated emission have the same energy, same frequency and are in phase. Therefore, all photons in the stimulated emission travel in the same direction. 

The number of photons emitted in the stimulated emission depends on the number of electrons in the higher energy level or excited state and the incident light intensity.

It can be written as:

Number of emitted photons α Number of electrons in the excited state + incident light intensity.

 Characteristics of Laser

Laser light has four unique characteristics that differentiate it from ordinary light: these are

1.       Coherence       2.  Directionality          3. Monochromatic                   4. High intensity

1.       Coherence

In laser, the electron transition occurs artificially. In other words, in laser, electron transition occurs in specific time. All the photons emitted in laser have the same energy, frequency, or wavelength. Hence, the light waves of laser light have single wavelength or color. Therefore, the wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated emission is used to produce light. 

Thus, light generated by laser is highly coherent. Because of this coherence, a large amount of power can be concentrated in a narrow space.

2.       Directionality

In conventional light sources (lamp, sodium lamp and torchlight), photons will travel in random direction. Therefore, these light sources emit light in all directions.

On the other hand, in laser, all photons will travel in same direction. Therefore, laser emits light only in one direction. This is called directionality of laser light. The width of a laser beam is extremely narrow. Hence, a laser beam can travel to long distances without spreading.

If an ordinary light travels a distance of 2 km, it spreads to about 2 km in diameter. On the other hand, if a laser light travels a distance of 2 km, it spreads to a diameter less than 2 cm.

3.       Monochromatic

Monochromatic light means a light containing a single color or wavelength. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths or colors. Therefore, ordinary light is a mixture of waves having different frequencies or wavelengths.

On the other hand, in laser, all the emitted photons have the same energy, frequency, or wavelength. Hence, the light waves of laser have single wavelength or color. Therefore, laser light covers a very narrow range of frequencies or wavelengths.

4.       High Intensity

You know that the intensity of a wave is the energy per unit time flowing through a unit normal area. In an ordinary light source, the light spreads out uniformly in all directions.

In laser, the light spreads in small region of space and in a small wavelength range. Hence, laser light has greater intensity when compared to the ordinary light.

Types of lasers

Lasers are classified into 4 types based on the type of laser medium used:

1.       Solid-state laser   2. Gas laser             3. Liquid laser            4. Semiconductor laser

Applications of Lasers

The most significant applications of lasers include:

a.       Lasers in Medicine

I.                    Lasers are used for bloodless surgery.

II.                  Used in welding the retina of the eye

III.                Lasers are used for boring hole in the skull.

IV.                Lasers are used in cancer diagnosis and therapy.

V.                  Lasers are used in fiber-optic endoscope to detect ulcers in the intestines.

VI.                The liver and lung diseases could be treated by using lasers.

VII.              Lasers are used to study the internal structure of microorganisms and cells.

VIII.            Lasers are used to produce chemical reactions.

IX.                Lasers are used to create plasma.

X.                  Lasers are used to remove tumors successfully.

XI.                Lasers are used to remove the caries or decayed portion of the teeth [Dentistry]

XII.              Lasers are used in cosmetic treatments such as acne treatment, cellulite and hair removal.

b.      Lasers in Communications

I.                    Laser light is used in optical fiber communications to send information over large distances with low loss.

II.                  For hologram production

III.                CD, DVD, VCD playing.

IV.                Laser light is used in underwater communication networks.

V.                  Lasers are used in space communication, radars and satellites.

c.       Lasers in Industries

I.                    Lasers are used to cut glass and quartz.

II.                  Lasers are used in electronic industries for trimming the components of Integrated Circuits (ICs).

III.                Lasers are used for heat treatment in the automotive industry.

IV.                Laser light is used to collect the information about the prefixed prices of various products in shops and business establishments from the bar code printed on the product.

V.                  Ultraviolet lasers are used in the semiconductor industries for photolithography. Photolithography is the method used for manufacturing printed circuit board (PCB) and microprocessor by using ultraviolet light.

VI.                Lasers are used to drill aerosol nozzles and control orifices within the required precision.

d.      Lasers in Security

I.                    A laser helps in guidance system of missiles, aircraft and satellite .

II.                  With the help of generation of isotopes for nuclear weapon/ reactor/ enriched uranium production.

e.      Laser in Science and Technology

i.         With the help of a laser, it is possible to count the number of atoms in a substance.

  ii. Lasers are used to measure the pollutant gases and other contaminants of the atmosphere.

III.                Lasers helps in determining the rate of rotation of the earth accurately.

IV.                Lasers are used in computer printers.

V.                  Lasers are used for producing three-dimensional pictures in space without the use of lens.

VI.                Lasers are used for detecting earthquakes and underwater nuclear blasts.

VII.              A gallium arsenide diode laser can be used to setup an invisible fence to protect an area.

f.        Lasers in Military

I.                    Laser range finders are used to determine the distance to an object.

II.                  The ring laser gyroscope is used for sensing and measuring very small angle of rotation of the moving objects.

III.                Lasers can be used as a secretive illuminators for reconnaissance during night with high precision.

IV.                Lasers are used to dispose the energy of a warhead by damaging the missile.

V.                  Laser light is used in LIDAR’s to accurately measure the distance to an object.

PRESENTATION

Step I: The teacher revises the previous topic.

 Step II: The teacher explains the acronym LASER.

Step III: The teacher explains the principle of working of LASER.

Step IV: The teacher explains characteristics of LASER.

Step V: The students chorus the  characteristics of LASER

Step VI: The teacher explains the application of LASER .

EVALUATION:

The teacher evaluates the lessons by asking the following questions:

1.       State the meaning  of the acronyms LASER.

2.       List the principles of working of a LASER.

3.       Mention characteristics of LASER

4.       State the application of LASER in medicine and industry.

 ASSIGNMENT

 What are the dangers involved in using LASER.