 Hello everyone, myself Dr. Ms. Tejasvis Rirangagharke, working as an assistant professor in the Department of Humanities and Science, Wolchen Institute of Technology, Solapur. I welcome all of you in the session of Physics. Today we will see the basic concept of polarisation. At the end of the session, students will be able to understand the difference between polarised and unpolarised light, plane of vibration and plane of polarisation, isotropic and anisotropic medium and calcite crystal. These are the content of the session. Let us see what is polarisation. The transverse nature of light proves the polarisation property. Light coming from common light source is unpolarised light. It can be transformed into different types of polarisation by using different optical devices. The state of polarisation cannot be recognised by our eyes. The knowledge of polarisation is essential for understanding the propagation of electromagnetic waves propagating through waveguides and optical fibres. Polarisation is an important physical property of electromagnetic waves which is connected with a travelling character with respect to the direction of propagation of electric and magnetic field vector. Let us see what is unpolarised light. Unpolarised light is a light which oscillates in more than one plane with equal probability at right angles to the direction of propagation. Think for a while. Can you tell any one example of unpolarised light? A phenomenon in which light is restricted to vibrate in only one plane is called polarised light. The polarised light is not produced naturally. It is obtained by converting natural light into polarised light by using optical elements. Polarised light consists of waves with planes of vibration in a single plane normal to the ray of direction. It may be regarded as the resultant of two mutually perpendicular coherent waves having a zero phase difference. Polarisation of light describes the shape and locus of waves in which the plane perpendicular to the direction of propagation at a given instant of time and space. Depending upon the locus of waves, light may exhibit three different states of polarisation, plane or linear polarisation, elliptical polarisation and circular polarisation. Unpolarised human eye cannot identify the state of polarisation of a light. However, some insects and animals possesses polarisation-sensitive vision. In plane polarised waves have an oscillations which are strictly confined to a single plane perpendicular to the direction of wave propagation. The plane polarised wave is also called as linearly polarised wave. In circularly polarised light there is no preference to specific direction of oscillation. A circularly polarised light is the resultant of two coherent linearly polarised waves of equal amplitude oscillating in mutually perpendicular planes and are out of phase by 90 degree. An elliptically polarised light wave may be regarded as a resultant wave produced due to superposition of two coherent linearly polarised waves having different amplitudes, oscillating in mutually perpendicular planes and are out of phase. Diagram shows the unpolarised light, linearly, circularly and elliptically polarised light in which vibration occurs is known as plane of vibration and the plane contains the direction of vibration and the direction of propagation. The plane normal to the vibrational light is plane of polarisation and it contains only direction of propagation of light but not the vibrations. When ordinary light is passed through a tourmaline crystal, the light is polarised and the vibrations are confined only in one direction that is perpendicular to the direction of propagation of light. It describes the plane of vibration and plane of polarisation. Now let us see in detail what is isotropic and anisotropic medium. Isotropic medium is a homogeneous medium. The atoms in the crystal are arranged in a regular periodic manner. Refractive index is same in all the directions. The velocity of light of waves is same in all the directions. When the light is incident on an isotropic medium, it propagates as a single refracted wave. Glass, water, air, these are the examples. Anisotropic medium is a heterogeneous medium. The arrangement of atoms differs in different directions and planes. Hence the physical properties varies with direction. Refractive index is different along different direction. The force of interaction between electron cloud and the lattice is different in different directions. Hence the natural frequency of electron cloud is dependent on the direction it is vibrating. It resists in different velocities in different directions. When light is incident on an isotropic medium, it propagates as a double refracted ray. Calcite, ice, tourmaline, these are the examples. The figure shows the two materials isotropic and anisotropic. When the light is in isotropic medium, it propagates as a single wave. And when it enters in an anisotropic medium, it propagates as two waves. Anisotropic crystals are again classified as uniaxial and biaxial. In uniaxial crystal, one of the refracted ray is ordinary and second is extraordinary. Calcite, quartz, tourmaline, these are the examples. While in biaxial crystal, both the refracted rays are extraordinary. Maika, topaz, these are the examples. Now let us see in detail the structure of calcite crystal. Calcite is a common naturally occurring substance. A large calcite crystal is colorless and transparent. It is also called as Iceland Bar. Naturally occurring calcite crystal has a rhombohedral structure. Calcite is crystallized calcium carbonate, CaCO3. It is uniaxial crystal. It is bounded by six parallelograms. At the opposite corners, the three angles of 102 degree and at the other corner one angle of 102 degree and two angles of 78 degree. What is blunt corner? The corners where the three obtuse angles meet are known as blunt corners. Why? Optic axis is a line bisecting any of the blunt corners. Any line parallel to this line is an optic axis. Hence optic axis is a direction and not a specific line in the crystal. Let us see what is principal section. A plane containing optic axis and a plane perpendicular to the pair of opposite faces of a crystal is called as principal section for that pair of face. A calcite crystal has six faces and three principal sections. A principal section always cuts the surface of calcite crystal in parallelograms having an angle 78 degree and 102 degree. This is the detailed structure and information regarding calcite crystal as uniaxial crystal. These are the references related to the session. Thank you.