Short Notes on Lasers(physics)

LASER(Light Amplification by Stimulated Emission of Radiation)

• A laser is a very special source of light and it generates optical radiation utilizing a process called stimulated emission.
• The basic principles of stimulated emission process were enunciated by Albert Einstein as early as 1917.
• The first laser  system was designed and fabricated by Theodore Maiman in the Hughes Research Laboratories, USA in 1960. This heralded a major revolution in the field of science and technology.
• Soon laser action was demonstrated in gases, liquids and other optical materials as well as in semiconductors.
• Today lasers come in all sizes and shapes.
• Some gives only nanowatts (10-9 watts) of power while others produce several terawatts (1012 watts) of power. Some operate continuously while a few others produce pulses which lasts only for a few femtoseconds (10-15 seconds).
• Laser is a unique source of light that differs, fundamentally from ordinary sources of light.
• The light generated by a laser is a coherent one where as ordinary light that we are familiar with is non coherent. The electromagnetic waves in a coherent radiation are all in phase, i.e., the different waves are all synchronized with respect to each other. This makes laser light extremely intense and highly directional.
• Laser light is also highly monochromatic, i.e., it contains essentially a single colour or wave length. Thus laser light is endowed with extraordinary properties.

LASER’S APPLICATIONS

• Because of the extremely high intensities laser beam can melt and evaporate any material onto which it is focussed. This makes it a very useful tool for cutting, drilling and welding applications which require great precision.
• Also the high intensities generate several unique nonlinear optical effects which can be induced only with the help of appropriate laser sources. One such nonlinear phenomenon is the second harmonic generation in which the frequency of the incident laser radiation gets doubled.
• For example when 1.06 nm wavelength (near infrared) radiation from a Neodymium YAG laser is made to pass through a potassium dihydrogen phosphate crystal, green radiation at 532 nm wavelength is generated under appropriate conditions.
• Lasers can induce several other nonlinear phenomena like optical phase conjugation, up conversion, parametric oscillations and stimulated Raman and brillouin scattering.
• Apart from these extensive use in basic and applied sciences as a unique of coherent radiation, lasers find widespread use in industry, medicine, communication, defense etc.
• In fact there is hardly any area of human activity that remains unaffected by laser revolution.
• Laser surgery has become popular in the medical field as it has several advantages over conventional surgical method. It can also be used as a bloodless scalpel and the wounds heal must faster in this case. Lasers are used extensively now for correcting the various types of defects in the human eye.
• Tiny semiconductor diode lasers together with very thin optical fibers are currently revolutionizing the entire field of communication technology.
• Modulated laser light passing through these hair-thin glass fibers can carry enormous amount of information of all kind. Thus lasers are helping to shape the information society of tomorrow. Lasers play a major role in defense also.
• Lasers range finders, laser guided weapons and missiles have been found to be very effective in the actual field use. Laser based nonlethal weapons that temporarily cause blindness to the enemy are now being tested. In the Star War project of the USA, high power ground based lasers were employed to destroy the enemy missiles and satellites.
• Lasers also have wide-spread use in numerous applications like laser printing, CD player, barcode reader etc to name a few.

ATOMIC STRUCTURE AND ENERGY LEVEL

• Thomson Empirical Model
  • The model could explain the stability and neutrality of atom but couldn’t explain the discrete emission of radiations and large angle scattering of α particles by foil.
• Rutherford Scattering Formula
  • The number of α-particles scattered at an angle θ by a target is given by

• N₀ = no. of incident a-particles
  • n = number of target atoms per m³
  • t = thickness of target
  • ze = charge on nucleus
  • = charge of a-particles
  • r = distance of screen from target E
  • k = initial kinetic energy of incident particle
• Bohr’s Atomic Model
  • Specified orbits satisfying condition 
  • Coulombean force between electron and nucleus supplies centripetal force. 
  • Transitions from high energy level to lower energy level emits a spectral line of frequency v given by 
  • Velocity of electrons  

• Energy of electrons in the nth orbit

• Wavelength of radiation in the transition 

 Where constant R called Rydberg’s constant and has the value 1.097 x 10⁷ m^-1

• Hydrogen Spectrum
  • The spectrum of hydrogen consist of five series of lines.

• Modifications of Bohr Theory
• It was found that four quantum numbers were needed to completely specify the quantum state of an electron.
  • Principal Quantum number (n)
  • Orbital (Azimuthal) quantum number (t)
  • Orbital magnetic quantum number (m₁)
  • Spin and spin magnetic quantum numbers (s and m_s)
• Pauli Exclusion Principle
  • No two electrons in an atom can have the same values of all the four quantum numbers.
• X-Rays
  • X-rays are electromagnetic radiations having wavelength from a fraction of an Angstrom (A) to about 100 A.
• Production
  • The electrons emitted from the heated filament are accelerated towards a copper anode under a high

 Potential difference. A target, which is a metal of high atomic number and high melting point is embedded on the anode. The intensity of the X-ray beam is controlled by the filament current which determines the number of electrons striking the target per unit time.

• Diffraction of X-rays by crystals

 

• Cathode Rays
  • They travel in straight lines and cast shadows of object placed in their path and can transfer their energy. When cathode ray strike a solid substance of large atomic weight, they produce X-rays.
  • For cathode rays

• Photoelectric Effect
  • Emission of electrons from the surface of a metal, when electromagnetic radiation of high enough frequency falls on it, is called the photoelectric effect.

• This is called Einsteen’s Photoelectric equation.
  • v = frequency of the radiation
  • hv = energy of photon
  • h = Planck’s constant having value 6.625 x 10^-34 Js
  • The threshold frequency v₀ is
  • hv₀ = W

E_max = h (v –v₀)

• Compton Effect
  • When X-rays are scattered by free electrons, their wavelength increases. This phenomenon is called Compton effect.
• Change in wavelength 

θ = angle of scattering, λ = wavelength of incident photon λ′ = wavelength of scattered photon m₀ = mass of electrons C = speed of light