A Nicol prism can be used as a polariser and an analyser. When two Nicol prisms are placed co-axially as shown in Figure 6.12, then Nicol prism Pacts as polariser and Nicol prism A acts as analyser. Such an arrangement is known as polariscope.
When principal section of both the Nicols are parallel, then emitted E-ray from polarizer P has vibrations parallel to principal section of analyser A, so get freely transmitted through it. In this setting of Nicols the intensity of emitted light is maximum. This position and the position when the angle between the principle sections of two prisms is 180° is known as “Parallel nicols
When Nicol A is rotated from its position, intensity of light emitted from it decrease
and becomes zero when principal sections of two planes at right angle to each other. In this situation light emitted from polarising Nicol P has vibration in a plane normal to principal section of analysing Nicol A and is totally internally reflected back from Canada Balsam layer and no light is emitted. In this setting, two Nicols are said to be “Crossed Nicols”.
For all other intermediate positions between parallel and crossed, the E-ray emitted by polariser falls on analyser and get splitted into two components: one having vibrations in the principal section of analyser and other having vibrations perpendicular to principal section of analyser. The latter is totally internally reflected by Canada Balsam layer while the former is freely emitted. The intensity of emitted light is given by Malus law i.e., I a cos2 θ, where e is angle between principal sections of analyser or polariser.