Diffraction and polarization of light

3) Resolving power

  • When two objects are very close to each other they may appear as one and we might not see them as separate objects just by magnifying them

  • To separate two objects which are very close together, optical instrument such as telescope ,microscope,prism,grating etc are employed

  • The separation of such close object is termed as resolution and the ability of an optical instrument to produce distinctly separate images of two close objects is called its resolving power

  • Every optical instrument has a limit up to which it can produce distinctly separate images to two objects placed very close to each other

  • After detailed study of the intensity of diffraction pattern of two very close point objects ,lord Rayleigh suggested that the two objects will be just resolved when central maximum of diffraction pattern of first object lies on first secondary minimum of diffraction pattern of second object

  • The minimum distance between two point object which can just appear to be as separate by optical instrument is called the limit of resolution of the instrument

  • we will discuss about the resolving power of optical instrument when we study exclusively about optical instruments

4) Polarization of light

  • Waves are generally of two types

    i) Longitudinal waves: In case of longitudinal waves ,particles of the medium oscillates along the direction of the propagation of the waves
    ii) Transverse waves: In this case direction of oscillation of particles is perpendicular to the direction of propagation of waves

  • we already know that light is an EM wave in which electric and magnetic field vector vary sinosoidally,perpendicular to each other as well as perpendicular to the direction of propagation of light wave

  • This shows that light waves consists of transverse waves
  • The fact that light consists of transverse waves can be confirmed in the experiments in which beams of lights were allowed to pass through Polaroid which are artificial crystalline materials that allow lights vibrations to pass through only in a particular plane

  • We would now observe the light passing through two Polaroid A and B placed one behind another in from of source of light as shown below in the figure

    Polarization of light
  • when axes a and b of Polaroid A and B respectively are parallel to each other then the light through Polaroid B appears slightly darker intensity of light is reduced after being transmitted from Polaroid A

  • Since axis of both the Polaroid are parallel to each other so light transmitted from Polaroid A is transmitted as it is by Polaroid B
  • Now if start rotating Polaroid B about the z-axis ,one will observe the variation of intensity i.e. light passing through crystal B becomes darker and darker and disappears at one stage

  • This happens when axis a of Polaroid A is perpendicular to axis b of the Polaroid B as shown in fig 3(b)

  • Again on rotating Polaroid B in same direction light reappears and becomes brightest when the axis a and b are again parallel

  • This simple experiment proves that light consists of transverse waves

  • Here in this experiment Polaroid A acts as polarizer and the beam transmitted through polarizer is linearly polarized and second Polaroid acts as analyzer

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