 Jason �� ආhausු ල�は මාun වනතු අජලටු ක්ණාත් ලාu ධඪු තු්බුම නො Nok෉ඉඏරීක ක්ණාතී ක්ණතුȚ, � whopping�නුම් මබ දිකෙරලාu න්ඹතුමු බෙඟ මෙතුඵෙau හොි තිකානු එයිවක් එයි ක්ත� difference between ordinary and extraordinary range, difference between positive and negative crystal and about the malice law. Now at the end of session the students will able to understand the concept of optical activity, specific rotation, half shade polarometer and applications of polarization. These are the content of the session, what is the optical activity. Certain crystals and solution possess a natural ability to rotate the plane of polarization about the direction of propagation. In case of crystal this ability arises due to the twisted arrangement of atomic layers with respect to one another. In liquids and solutions the optical activity is due to certain structural symmetry in their molecules. The optical activity is found to be bigger organic molecules provide a number of clues which helps us to understand the biological activity. Optical activity is the ability to rotate the plane of polarization of plane polarized light when passed through a certain substance. The substance which have an ability to rotate the plane polarized light passing through them are called as optical active substance. Cots, Cina bar are the optical active crystals, terpentine tartaric acid are the optical active liquids and aqueous solution of sugar is optically active solution. These are the examples of optical active substance, solid, liquid and solutions. Optically active substances are classified into two types, dextrorotatory and leorotatory. In dextrorotatory the substance which rotate the plane of polarization in clockwise direction or to the right side. In leorotatory the substance which rotate the plane of polarization in anticlockwise direction or to left. Optical active substances are classified into two different types, dextrorotatory and leorotatory. In dextrorotatory the substance which rotate the plane of polarization in clockwise direction or to the right. In leorotatory the substance which rotate the plane of polarization in anticlockwise or to the left. What is the specific rotation? The specific rotation is a major of optical activity of a sample. It is the rotation produced for 1 mm flap for a solid or 100 mm path length for a liquid. If an optical active material escapes between two crossed polarizers, the field of view becomes bright. In order to get darkness, the analyzer has to be rotated through an angle. The angle through which analyzer is rotated equals to the angle through which plane of polarization is rotated by an optical active substance. This angle depends on thickness of substance, density of material or concentration of solution, wavelength of light and temperature. In other words, specific rotation is the rotation caused by one decimeter length of a solution containing one gram of optical active substance per centimeter cube of the solution. These are the two formulas from which we can calculate the specific rotation value. The optical active phenomenon, the unpolarized source, when passed through a polarizer, the restriction is only along one plane and we are getting plane polarized light. This plane polarized light is passed through a tube containing optical active sample. Then there is a change in rotation of plane polarized light which is detected under the detector. Now let us see the construction of Lorentz half shade polarometer. What is polarometer? Polarometer is an instrument used to determine the optical rotation of solutions, but when it is used in determining the quantity of sugar in the solution, it is called as saccharometer. The half shade plate consists of the half part made up of quartz crystal and the half part is made up of the glass crystal. This polarometer was invented by scientist Lorentz, hence it is called as Lorentz half shade polarometer. Now let us see in detail the construction of polarometer instrument. A polarometer consists of a glass tube for holding the solution which is under test and it is held between the crossed nickel prism. Beyond the polarizing nickel prism, a half shade plate is located which is used for accurately adjusting the two nickel prism for crossed position. Light from the monochromatic source is made parallel by the lens L and then incident on the polarizer N1. The light coming out from the polarizer is plane polarized light. The polarized beam is then passed through a liquid, passed through a half shade plate and then through a glass tube G which contains the solution. The light emerging from the solution will be incident on the analyzer N2. The light is observed through a telescope T and the analyzing nickel prism N2 can be rotated about the axis of tube and the rotation can be measured with the help of scale. What is the function of half shade plate? The half shade plate consists of the two materials glass and quartz. The glass material is isotropic medium and the quartz material is an isotropic medium. In isotropic medium, when the incident light is refracted in only one direction, while in quartz crystal, the refract incident ray will be reflected as a two refracted ray, ordinary and extraordinary ray having a phase difference of 180 degree. In the glass, incident ray will be act as a same but in the quartz it will be splits into two rays. Both the rays are when they are parallel to the optic axis, we are getting the equal illumination on both the sides while during the rotation with phase difference of 180 degree we are getting the one bright and one dark region and the vice versa process will be continued unless and until the rotation of two rays ordinary and extraordinary ray. Then plot the graph of theta versus c and do the calculation by using the corresponding formula for the calculation of specific rotation. Thing for while, half shade plate is made from which material. Now let us see the applications of plain polarised light. It is useful in the three dimensional movies. Polarised light is used in saccharimeter which are used for determining the quantity of sugar in a solution. Polarised use of polarisation phenomenon is used in liquid crystal display, wrist watches, calculators, TV screens, etc. Polarised are used to control the intensity of light entering in the trains and aeroplanes. These are the some applications of plain polarised light in polarisation. These are the references for the session. Thank you.