polarization is measured as a function of wavelength of light, or broad-band aperture polarimetry. Measuring optical rotation Optically active samples, such as solutions of chiral molecules, often exhibit circular birefringence. Circular birefringence causes rotation of the polarization of plane polarized light as it passes through the sample. In ordinary light, the vibrations occur in all planes perpendicular to the direction of propagation. When light passes through a Nicol prism its vibrations in all directions except the direction of axis of the prism are cut off. The light emerging from the prism is said to be plane polarised because its vibration is in one direction. If two Nicol prisms are placed with their polarization planes parallel to each other, then the light rays emerging out of the first prism will enter the second prism. As a result, no loss of light is observed. However, if the second prism is rotated by an angle of 90°, the light emerging from the first prism is stopped by the second prism and no light emerges. The first prism is usually called the polarizer and the second prism is called the analyser. A simple polarimeter to measure this rotation consists of a long tube with flat glass ends, into which the sample is placed. At each end of the tube is a Nicol prism or other polarizer. Light is shone through the tube, and the prism at the other end, attached to an eye-piece, is rotated to arrive at the region of complete brightness or that of half-dark, half-bright or that of complete darkness. The angle of rotation is then read from a scale. The same phenomenon is observed after an angle of 180°. The specific rotation of the sample may then be calculated. Temperature can affect the rotation of light, which should be accounted for in the calculations. where: [α]λT is the specific rotation. T is the temperature. λ is the wavelength of light. α is the angle of rotation. l is the distance the light travels through the sample, the path length. is the mass concentration of solution. See also Ellipsometry References External links Polariscope – Gemstone Buzz instrument to measure optical properties. EU Project NanoCharM nanocharm.org Polarization (waves) Optical metrology