A polariscope is an instrument used primarily to gauge the state and direction of stress at the point in a body (photoelastic sample). It uses a combination of polaroid sheets that polarize the incident light waves. One sheet is called a polarizer while the other in-line is called the analyzer. My question is " When the two are crossed that is at 90 degrees the output is extinction. i.e. when the two sheets have axis of polarisation in perpendicular direction, the light wave is absorbed completely. However at other angles some light seems to come out of the analyzer. Why is this so? If the polarizer works perfectly i.e., polarizes the light, then there should be no illumination except at the parallel orientation. But in actual practise it is found that there is maximum illumination in parallel orientation and then decreasing illumination till the two are crossed, when the illumination is totally zero. What leads to partial illumination?"