Malus' Law


To demonstrate how the intensity changes in a polarized and analyzed light beam.Laser cart, polarizer and analyzer with carefully calibrated angular positioning, radiometer, TV camera with 20cm auxilliary lens and monitor.

Setup Time

5 min.




For a polarized light beam which is then analyzed by rotating a second polaroid in front of the first, the intensity is proportional to the square of the cosine of the angle between the polarizer and the analyzer. This is known as Malus' law. The laser beam is polarized by the first polaroid, and the intensity is measured using a radiometer as the analyzing polaroid is rotated from 0 degrees to 90 degrees. Actually, the laser beam is partially polarized, so the first polaroid is just completing the job.


Marcelo Alonso, A Simple Proof of Malus' Theorem in Geometrical Optics, AJP 16, 462-463, (1948). A. F. Leung, The Law of Malus Using Polaroid Polarizers, TPT 18, 612-614, (1980).