Malus's law reflects direct dependence between intensity of natural light and intensity of the linear polarized light which passed through special polaroids. They are made of tourmaline crystals.
Light polarization
It is known that light represents a cross electromagnetic wave. Electromagnetic oscillations are carried out by vectors of electric (E) and magnetic (H) of fields. The vector of electric field is called still light. It determines the size of the energy postponed in space. The intensity of light depends on the module of this vector.
Each of them makes fluctuations in the planes perpendicular to the plane of a vector of distribution of a wave. If these fluctuations are carried out in all directions (the perpendicularity of the planes remains), light is called unpolarized or natural. Such light waves are radiated by the Sun and all terrestrial sources.
The polarized light arises when passing by a wave of certain substances. The light vector begins to make fluctuations only in one plane which is perpendicular to the plane of fluctuations of a magnetic vector and a vector of the direction of distribution. Such light is called linearly - or plane-polarized. For a human eye it differs in nothing from natural, but by means of it it is possible to observe the interesting phenomena.
Malus's law
It is possible to receive plane-polarized light by means of a tourmaline crystal. In 1809 the French engineer E. Malus opened interesting property of such light. In the experiments he used two plates made of tourmaline. It placed a light source and these two plates on an optical bench. Malus placed plates so that it was possible to change a corner between them (it is formed by their planes of polarization). The plate located closer to a source began to be called a polarizer, and that that further - the analyzer. These names are conditional as plates have no qualitative distinction. When changing a corner the intensity of light which passed through the analyzer changed. If the planes of polarization were perpendicular, it was equal to zero. Each plate "cut out" certain planes of fluctuations of a light vector because of what the intensity of a light wave changed. After the careful analysis of the received results the formula connecting intensity of the plane-polarized light which passed through the analyzer with intensity of natural light was open. It looks so: I=0.5*I0 * (cosf) ^2, where I - intensity of natural light, I0 - intensity of light which passed the analyzer F - a corner between the planes of polarization of tourist's crimson plates.