Plarization

 Polarized light waves are light waves in which the vibrations occur in single plane. The process of transforming unpolarized light into polarized light is known as polarization of light. 

a

        From the phenomena of interference and diffraction of light, it is proved that light has a wave nature. However, this does not tell about the type of waves. Light waves are electromagnetic waves in nature that travel through the vacuum. There is a periodic fluctuation in electric and magnetic fields along the propagation of light wave. These fields vary at right angles to the direction of the propagation of the light wave, so light wave is transverse wave. Transverse nature of light make it possible to produce and detect polarized light.

      In transverse mechanical waves, such as produced in a stretched string, the vibrations of the particles of the medium are perpendicular to the direction of propagation of the waves. The direction can be oriented along vertical, horizontal or any other direction as shown in figure given below.

   Fig   Transverse waves on a string polarized (a) in a horizontal plane (b) in a vertical plane.

      In each of these cases, the transverse mechanical wave is said to be polarized. The plane of polarization is the plane containing the direction of the vibration of the particles of the medium and direction of propagation of the wave.

       A light wave produced by oscillating charge consists of a periodic variation of electric field vector accompanied by the magnetic field vector at right angle to each other. Ordinary light has components of vibration in all possible planes. Such a light is unpolarized On the other hand, if the vibrations are confined only in one plane, the light is said to be polarized.

Production and Detection of Plane Polarized Light.

      An ordinary incandescent light emits unpolarized light, as does the sun, because its (electrical) vibrations are randomly oriented in space. It is possible to obtain plane polarized beam of light from unpolarized light by removing all waves from the beam except those having vibrations along one particular direction. This can be achieved by various processes such as selective absorption, reflection from different surfaces, refraction through crystals and scattering by small particles.

      The selective absorption method is the most common method to obtain plane polarized light by using certain types of materials called " dichroic substances ". These materials transmit only those waves, whose vibrations are parallel to a particular direction and will absorb those waves whose vibrations are in other directions. One such commercial polarizing material is a Polaroid.
      If unpolarized light is made incident on a sheet of Polaroid, the transmitted light will be plane polarized. If a second sheet of Polaroid is placed in such a way that the axes of the Polaroid, shown by straight lines drawn on them, are parallel (figure below), the light is transmitted through the second Polaroid. If the second Polaroid is slowly rotated about the beam of the light, as axis of rotation, the light emerging out of the second Polaroid gets dimmer and dimmer and disappears when the axes become mutually perpendicular. The light reappears on further rotation and becomes brightest when the axes are again parallel to each other.

  Fig   Experimental arrangement to show that light waves are transverse.

        This experiment proves that light waves are transverse waves. If the light waves were longitudinal, they would never disappear even if the two Polaroids were mutually perpendicular.
        Reflection of light from water, glass, snow and rough road surfaces, for larger angles of incidences, produces glare. Since the reflected light is partially polarized, glare can considerably be reduced by using Polaroid sunglasses.
      Sunlight also becomes partially polarized because of scattering by air molecules of the Earth's atmosphere. This effect can be observed by looking directly up through a pair sunglasses made of polarizing glass. At certain orientations of the lenses, less light passes through than at others.

Optical Rotation

      When a plane polarized light is passed through certain crystals, they rotate the plane of polarization. Quartz and sodium chlorate crystals are typical examples, which are termed as optically active crystals.
     A few millimeter thickness of such crystals will rotate the plane of polarization by many degrees. Certain organic substances, such as sugar and tartaric acid, show optical rotation when they are in solution can be used to determined their concentration in the solutions.

  Fig   Solution of an optical isomer rotates the plane of polarization of incident light so that it is no  longer horizontal but at an angle. The analyzer thus stops the light when rotated from the vertical (cross) positions.

Video for Polarization of Light

Diffraction of X-Rays by Crystals

X-rays are diffracted by crystals in a manner dependent on the wavelength of the rays and the space lattice of the crystal. Thus X-ray diffraction provides a means for study of the structure of crystalline substances, or of substances which have crystalline phases. The method adopted depends on the form in which the substance is available. With large crystals Laue diagrams can provide useful characterization, but more frequently the crystal is rotated when mounted at the center of a cylindrical film, thus bringing successive sets of crystalline planes into position. The Debye-Scherrer ring or powder method is used when the specimen consists of a number of small crystals. Because of the number of crystals, randomly distributed, some are usually available in each plane to the diffract the X-ray beam.

a

        X-rays is a type of electromagnetic radiation of much shorter wavelength, about m. In order to observe the effects of diffraction, the grating spacing must be of the order of the wavelength of the radiation used. The regular array of the atoms in a crystal forms a natural diffraction grating with spacing that is typically m. The scattering of X-rays from the atoms in a crystalline lattice gives rise to diffraction effects very similar to those observed with visible light incident on ordinary grating.

       The study of atomic structure of crystals by X-rays was initiated in 1914 by W. H. Bragg and W. L. Bragg with remarkable achievements. They found that a monochromatic beam of X-rays was reflected from a crystal plane as if it acted like mirror. To understand this effect, a series of atomic planes of constant inter planer spacing d parallel to a crystal face are shown by lines PP', P1P1', P2P2' and so on, in the figure given below:

   Fig    Diffraction of X-rays from the lattice plane of crystal

      Suppose an X-rays beam is incident at an angle θ on one of the planes. The beam can be reflected from both the upper and the lower planes of atoms. The beam reflected from lower plane travels some extra distance as compared to the beam reflected from the upper plane. The effective  path difference between the two reflected beams is 2dsinθ. Therefore, for reinforcement, the path difference should be an integral multiple of the wavelength. Thus

2dsinθ =nλ

      
       The value of n is referred to as the order of reflection. The above equation is known as the Bragg equation. It can be used to determine inter planar spacing between similar parallel planes of a crystal if X-rays of known wavelength are allowed to diffract from the crystal.

    X-ray diffraction has been very useful in determining the structure of biologically important molecules such as hemoglobin, which is an important constituent of blood, and double helix structure of DNA.