Friday, 2 June 2017

Notes on Polarization of Light Waves

A light wave is an electromagnetic wave that travels through the vacuum of outer space. Light waves are produced by vibrating electric charges. The nature of such electromagnetic waves is beyond the scope of The Physics Classroom Tutorial. For our purposes, it is sufficient to merely say that an electromagnetic wave is a transverse wave that has both an electric and a magnetic component.

Polarization of Light Waves

  • Each atom produces a wave with its own orientation of 
  •  All directions of the electric field vector are equally possible and lie in a plane perpendicular to the direction of propagation
  • This is an unpolarized wave

  • A wave is said to be linearly polarized if the resultant electric field vibrates in the same direction at all times at a particular point 
  • Polarization can be obtained from an unpolarized beam by
    •  selective absorption
    • reflection
    • scattering

Polarization by Selective Absorption

  •  The most common technique for polarizing light
  •  Uses a material that transmits waves whose electric field vectors in the plane are parallel to a certain direction and absorbs waves whose electric field vectors are perpendicular to that direction
  • E. H. Land discovered a material that polarizes light through selective absorption
    •  He called the material Polaroid
    • The molecules readily absorb light whose electric field vector is parallel to their lengths and transmit light whose electric field vector is perpendicular to their lengths.
  •  The intensity of the polarized beam transmitted through the second polarizing sheet (the analyzer) varies as
    •  I = Io cos2 θ
      • Io is the intensity of the polarized wave incident on the analyzer
      • This is known as Malus’ Law and applies to any two polarizing materials whose transmission axes are at an angle of θ to ea􀄐h other

Polarization by Reflection

  • When an unpolarized light beam is reflected from a surface, the reflected light is
    • Completely polarized
    • Partially polarized
    • Unpolarized
  • It depends on the angle of incidence
    • If the angle is 0° or 90°, the reflected beam is unpolarized
    • For angles between this, there is some degree of polarization
    • For one particular angle, the beam is completely polarized
  • The angle of incidence for which the reflected beam is completely polarized is called the polarizing angle, θp
  • Brewster’s Law relates the polarizing angle to the index of refraction for the material.

Polarization by Scattering

  • When light is incident on a system of particles, the electrons in the medium can absorb and re-radiate part of the light
    • This process is called scattering
  • An example of scattering is the sunlight reaching an observer on the earth becoming polarized
  • The horizontal part of the electric field vector in the incident wave causes the charges to vibrate horizontally
  • The vertical part of the vector simultaneously causes them to vibrate vertically
  • Horizontally and vertically polarized waves are emitted.

Optical Activity

  • Certain materials display the property of optical activity
    • A substance is optically active if it rotates the plane of polarization of transmitted light
    • Optical activity occurs in a material because of an asymmetry in the shape of its constituent materials.

Liquid Crystals

  • A liquid crystal is a substance with properties intermediate between those of a crystalline solid and those of a liquid
    • The molecules of the substance are more orderly than those of a liquid but less than those in a pure crystalline solid
  • To create a display, the liquid crystal is placed between two glass plates and electrical contacts are made to the liquid crystal
    • A voltage is applied across any segment in the display and that segment turns on.

Liquid Crystals, 2

  • Rotation of a polarized light beam by a liquid crystal when the applied voltage is zero
  • Light passes through the polarizer on the right and is reflected back to the observer, who sees the segment as being bright

Liquid Crystals, 3

  • When a voltage is applied, the liquid crystal does not rotate the plane of polarization
  • The light is absorbed by the polarizer on the right and none is reflected back to the observer
  • The segment is dark

Liquid Crystals, final

  • Changing the applied voltage in a precise pattern can
    • Tick off the seconds on a watch
    • Display a letter on a computer display

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