A device, used in the polarization of light, that is placed in the eyepiece of a polarimeter to observe plane-polarized light. The analyser, which may be a Nicol prism or Polaroid, can be oriented in different directions to investigate in which plane an incoming wave is polarized or if the light is plane polarized. If there is one direction from which light does not emerge from the analyser when it is rotated, the incoming wave is plane polarized. If the analyser is horizontal when extinction of light takes place, the polarization of light must have been in the vertical plane. The intensity of a beam of light transmitted through an analyser is proportional to cos2θ, where θ is the angle between the plane of polarization and the plane of the analyser. Extinction is said to be produced by ‘crossing’ the polarizer and analyser.