Malus’s Law Definition

Malus’s law is a fundamental principle of optics that describes the behavior of polarized light. It is named after Étienne-Louis Malus, a French physicist who discovered it in 1808. According to the law, the intensity of polarized light passing through a polarizer is proportional to the square of the cosine of the angle between the polarizer and the direction of polarization of the light.

The Formula for Malus’s Law

The formula for Malus’s law is as follows: I = I0 cos² θ, where I is the intensity of the light after passing through the polarizer, I0 is the initial intensity of the unpolarized light, θ is the angle between the polarizer and the direction of polarization of the light. The law applies to all types of polarized light, including linearly polarized, circularly polarized, and elliptically polarized light.

Application of Malus’s Law

Malus’s law has many practical applications, including in the design of optical instruments such as polarizers, filters, and polarizing beam splitters. It is also used in the study of materials that exhibit polarization, such as liquid crystals, and in the analysis of the polarization of light emitted by astronomical objects, such as stars and galaxies.

Example of Malus’s Law in Action

A simple example of Malus’s law in action is the use of polarizing filters in photography. Polarizing filters are used to reduce glare and reflections in outdoor photography, such as on water or glass surfaces. By rotating the polarizing filter, the photographer can adjust the angle of polarization of the light entering the camera lens, and hence control the amount of light that passes through the filter. As the angle of the filter increases from 0° to 90°, the intensity of the light passing through it decreases according to the cosine² function described by Malus’s law.