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PGTRB:LATEST STUDY MATERIAL:PHYSICS:ELECTROMAGNETIC WAVES AND WAVE OPTICS.

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 PGTRB:LATEST STUDY MATERIAL:PHYSICS:ELECTROMAGNETIC WAVES AND WAVE OPTICS.

  • In 1864 Maxwell theoretically explained about the electromagnetic waves.
  • In 1888 Hertz produced electromagnetic waves using an oscillatory circuit.
  • The visible range of spectrum occupies a very small range of electromagnetic spectrum extendingapproximately3.8*10^-7m in the extreme violet region to 7.8*10^-7m in the extreme red.
  • Beyond the violet end of the visible spectrum are the ultraviolet rays, X-rays, and Gamma rays.
  • Beyond the red of the visible spectrum are the infra red, microwaves and radio waves.
  • Ozone layer blocks the harmful ultra violet radiations.
  • The heating up earth's atmosphere due to the infra red rays is called Green house effect.
  • The various theories which explain the nature of light are corpuscular theory by Newton, wave theory by Huygens, electromagnetic theory by Maxwell and Quantum theory by Planck.
  • Laws of reflection and Laws of refraction can be explained by the Huygens principle.
  • The dispersion of light through a prism gives rise to a spectrum.
  • Interference of light is due to the superposition of two coherent beams.
  • Sources that have a constant phase difference or no phase difference are called coherent sources.
  • Practically two coherent sources must be derived from a single source.
  • In Interference alternate dark and bright fringes of equal width are formed.
  • In Fresnel's biprism method, the virtual images are the two coherent sources.
  • In Lloyd's single mirror method, the source and its reflected virtual image are the two coherent sources.
  • Beautiful colours are seen when sunlight falls on thin films of oil on water.
  • When a ray of light travelling in a rarer medium is reflected by a denser medium, an automatic phase change of pi is introduced in the reflected ray.
  • If monochromatic light is used to form Newton's rings the Centre will be dark.
  • Concentric Newton's rings are produced due to interference.
  • Diffraction means bending of light at sharp edges or at narrow slits.
  • In Fresnel's diffraction the source and the screen are at finite distance and in Fraunhofer's diffraction the source and the screen art at infinite distance.
  • Diffraction grating is used to find out the wavelength of light.
  • In polarisation the vibrations are restricted to one plane.
  • Only transverse wave can be polarised.
  • If the vibration are restricted to on plane, then the waves are called plane polarised waves.
  • In a partially polarised light the amplitude of vibration is maximum in one plane and varies in all other planes.
  • In a plane polarised wave, the plane containing the vibration is called plane of vibration and any plane perpendicular to this is called plane of polarisation.
  • Polariser is a device used to produce plane polarised light
  • Analyser is a device used to find out whether the light is polarised or not.
  • Polariser can be used as an analyser and vice versa.
  • Pile of plates is a device used to produce polarised light due to reflection. 
  • The angle of polarisation for glass is 57.5 degree.
  • According to Brewster's law the refractive index of the medium is equal to the tangent of the angle of polarisation.
  • In double refraction, the ordinary ray of light obeys the laws of refraction and the extraordinary ray does not obey the laws of refraction.
  • Optic axis of a crystal is the direction in which the ordinary rays and the extraordinary rays travel with the same velocity.
  • Nicol prism is a good polariser as well as an analyser.
  • The electric and magnetic disturbances have the property of waves and propagate through free space without the help of material medium. They are called electro magnetic radiations.
  • In 1905 Einstein, by extending Planck's quantum theory of radiation suggested that light waves consists of small packets of energy or quanta also called photons.
  • Quantum concept of light alone can explain emission, absorption, and interaction of light with matter.
  • Michelson in  1926 gave an improved method using a multifaced mirror and employing long distance for the velocity of light.
  • The wave theory, light should travel with a greater velocity in a rarer medium than in a denser medium.
  • Michelson with improved apparatus later showed that the ratio of the velocity of light in air to the velocity of light in water is equal to the refractive index of water as predicted by the wave theory.
  • A beam of white light, when it passes through a prism is split up into its constituent colours and this splitting is called dispersion of light.
  • The refractive index for the material of a prism is different for different wave lengths.
  • The refractive index is more for violet rays of light than the corresponding values for red rays of light.
  • The deviation and the refractive index of the yellow rays are taken as mean value.
  • The ratio of angular dispersion for any two wavelength(colours) to the deviation of the mean wavelength is defined as the dispersive power of the material of the prism for those two wavelengths.
  • Y=Y1+Y2 This is known as the principle superposition.
  • The combined effect of superposition produces an increased intensity at some points and produces a decreased intensity at some other points. Such phenomenon is called Interference of light.
  • If two wave trains have the same wavelength and amplitude and start with the same phase, these two waves are said to be coherent.
  • In 1801, Thomas Young first demonstrated the interference of light by an experiment.
  • The bright bands are due to constructive interference and dark bands are due to destructive interference of light.
  • The distance between the two successive bright or dark bands is called the bandwidth.
  • The phenomenon of Polarisation of light clearly proves that light waves are transverse waves.
  • A device which produces polarised light is called a polariser.
  • The tangent of the polarising angle is numerically equal to the refractive index of the medium. This is known as Brewster's law.
  • Crystal like calcium, quartz, and tourmaline having only one optic axis are called uniaxial crystal.
  • Crystal like mica, topaz, selenite, having two optic axes are called biaxial crystals.
  • A polarised is a simple form of polariser, which is now being used for productive plane polarised light.
  • Polaroids like H-Polaroids and K-Polaroids are prepared in which a thin film of polyvinyl alcohol is used.




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