Polarized light has been widely used in metallographic research. Polarized light devices are used in both vertical and large metallographic microscopes made in China. Next, we introduce the basic knowledge of polarizing light in metallographic microscope and the application of polarizing elements.

The light waves emitted by an actual source are dengas of electromagnetic waves radiated by a large number of atoms (or molecules or ions). The electromagnetic theory and practice of light have proved that, as a luminous atom, it continuously emits electromagnetic waves with invariable azimuth of electric vibration vector E and magnetic vibration vector H within about 10-8 seconds. People often refer to the electric vibration vector E in the light wave as the light vibration vector, referred to as the light vector. The plane determined by the ray vector E and the ray direction is called the vibration plane of the ray vector. The light wave whose vibration plane is confined to a fixed azimuth is called plane-polarized light.

The light emitted by an atom in a real source within 10 to 8 seconds is plane-polarized light. One of the atoms in the light source stops emitting light and, after a hiatus to get enough of it, can light up again and still emit plane-polarized light as before. But in these two cases, the orientation of the vibration plane is generally different. Each atom in the light source goes on and off, off and on. The actual source of a metallographic microscope is made up of trillions of such atoms, which emit uneven, plane-polarized light in different directions of the vibrating plane.

In nature, common light sources such as the sun, electric lamp, carbon arc, etc., emit a large number of plane-polarized light with all vibrational orientation of the mixture. From the point of view of wave surface A, the vibration of light vector E is symmetric in all directions. These vibration directions exist at the same time or rapidly and irregularly replace each other. This light is called natural light. The vibrations of natural light in different directions do not differ from each other, and they are statistically symmetric with respect to light.