Abstract

This paper illustrates the use of Flash – a multimedia software that enhances teaching / learning of Science. In the present context, Flash is utilized to explain the principle of laser which is the abbreviation for Light Amplification by Stimulated Emission of Radiation. Laser is a coherent beam, obtained due to stimulated emission of radiation. Hence it involves the jumping of electrons from excited state to ground state, while emitting a coherent laser beam. The flash with its motion tweening capability introduces dynamic motion of electrons. Hence concepts like induced absorption, stimulated emission, and production of laser light which involve jumping of electrons are explained effectively. This project paper supported by Flash allows the student to learn in his own pace, thus making teaching/learning an interesting, interactive process.

Introduction

The word laser is the abbreviation for Light Amplification by Stimulated Emission of Radiation.

The laser is a device that produces a light beam with some remarkable properties.

Characteristics of Laser

The laser beam is

coherent with the waves exactly in phase with one another unlike ordinary light
monochromatic
does not diverge at all
is extremely intense.

Ordinary light Monochromatic Incoherent Monochromatic

Light Coherent Light

Figure 1: Types of Waves

Laser Action

The probability of an atom to radiate a photon is given by Einstein. According to him, the probability per unit time that the atom will radiate a photon by returning to its ground level is the sum of two terms

1.Spontaneous emission term http://geekum.esmartweb.com/spontaneous emission.swf

2.Stimulated emission term

Stimulated emission term is proportional to the number of photons with relevant energy present in the environment of the atoms. A photon produced by stimulated emission is always in phase with the stimulating photon. In a system of atoms in thermal equilibrium, the number of atoms in the ground state is much more than the number of atoms in the excited state, which is called the normal population. But for emission to take place we require large number of electrons in the excited state.

Normal Population Population Inversion

Figure 2 : Various States of Electron

An electron is also taken from ground state E₀ to the excited state E₁by a photon of energy E₁ - E_0.This is called stimulated or induced absorption. This can be achieved by any external agency like optical pumping, where a large number of atoms are first excited and pumped to a higher energy state. Now the number of atoms in the excited state is much more than the number of atoms in the ground state. This phenomenon is called population inversion.

Let us consider a system containing a large number of atoms in an excited energy state E₁ by the process of induced absorption. Let the spontaneous emission probability term be nearly zero and the stimulated emission term be large. But if there are no photons of energy E₁ - E₀ in the system, the atoms are unable to decay because the stimulated emission term is proportional to the number of photons present in the environment of the system.

Now if a few photons of energy E₁ - E₀ are introduced, the photons immediately stimulate the emission by equal number of photons of each of energy hv = E₁ - E₀. This increase in the number of photons in turn stimulates the emission further and the process continues to give a chain reaction or avalanche effect. This is called laser action. By this action all photons are in phase with each other to reinforce to give a very intense beam.

The phenomenon of laser action is explained in the following manner. There are a large number of ways to produce population inversion and one of them is optical pumping. Here an external light is used. Some photons have the desired energy to raise the atoms to excited states that decay spontaneously into the desired meta stable states where the life time of atom is 10^-3s instead of 10^-8s that prevail in normal excited states.

Atoms in the ground state are pumped to the state E₂by photons of energy hv¹ = E₂- E₀or by collisions

http://geekum.esmartweb.com/induced absorption.swf

Rapid transition to metastable state by spontaneous emission of photons

http://geekum.esmartweb.com/metastable state.swf

Metastable states occupied in many atoms.

Figure 3 : Metastable State

Induced emission occurs where photons of energy hv = E₁- E₀are incident with the secondary photons themselves inducing further transitions to produce an avalanche of coherent photons.

http://geekum.esmartweb.com/laser formation.swf

Conclusion

Thus to achieve laser action

There must be an inverted population, that is more atoms in an excited state than in the ground state
The excited state must be a meta stable state, because the life time of atoms is more in this state

3. The emitted photons must stimulate further emission. This is achieved by the use of reflective mirrors at the ends of the system.

Literature References

Andy Ashdown & Andrew Easton (2001). Installing Software. Dorling Kindersley Ltd, London, U.K.

Christian Crumlish & Malcolm Humes (1997). Web Publishing with Netscape for Busy People. Tata McGraw Hill Publishing Co., Ltd, New Delhi, India

Lewis, Chris, (1997). 101 Essential Tips Exploring Multimedia. Dorling Kindersley Ltd, London, U.K.