rly polarised C = log l (7.10) light, those most widely known being birefringence or double re- fraction, reflection, scattering and dichroism. Double reflection 1 = l e - kx (7.11) occurs in certain types of natural crystal such as calcite and quartz and will divide a beam of unpolarised light into two separate polarised beams of equal intensity. By eliminating one of the po- Note that in considering a medium of thickness X, is not the larised beams a very efficient linear polariser can be made. intensity of incident light due to there being some reflection at the Dichroic polarisers make up the great majority of commercially first surface. Similarly I is not the emergent intensity owing to produced synthetic polarisers. They exhibit dichroism, the property reflection at the second surface. By measuring the emergent intens- of absorbing light to different extents depending on the polarisation ity for two different thicknesses the losses due to reflection may be form of the incident beam. eliminated. The light emerging from a linear polariser can be given a 'twist' SO that the vibrations are no longer confined to a single plane but 7.10.3 Polarisation instead form a helix. This is achieved by inserting a sheet of double-refracting material into the polarised beam which divides For an explanation of polarisation of light we need to invoke the the beam into two beams of equal intensity but with slightly differ- wave concept and the fact that light waves are of a transverse nature ent speeds, one beam being slightly retarded. The light is said to be possessing transverse vibrations which have both an electric and circularly polarised. magnetic character. Figures 7.14 and 7.15 set out to illustrate the The application and uses of polarised light are considerable: liquid meaning of unpolarised and linearly polarised light. crystal displays, control of light intensity, blocking and prevention In Figure 7.14 a wave is propagating in the X direction with the of specular glare light, measuring optical rotation, measuring 