A polarizer is an optical filter that allows light waves of a specific polarization to pass through while blocking others. It transforms undefined or mixed polarization light into well-defined polarized light. There are linear polarizers and circular polarizers. Linear polarizers make light vibrate mostly in one direction, while circular polarizers create circularly polarized light. They find applications in various optical instruments, photography, and LCD technology. Polarizers can be designed for electromagnetic waves beyond visible light, like radio waves and X-rays. Circular polarizers are used in 3D glasses and have roles in photonic techniques such as ellipsometry and optical communication.