The interference and diffraction of light show that light is actually fluctuating. Whether it is a transverse wave or a vertical wave, it can produce interference and diffraction. Its generation method is mainly through refraction and reflection. It can also be generated by dichroism, crystal birefringence, etc.

If we let the rope pass through a fence, the propagation of the wave will be limited by the fence. If the direction of the fence gap is consistent with the vibration direction, the wave can pass through the fence smoothly. If the gap direction is perpendicular to the vibration direction, the wave is blocked and can not continue to travel forward. This is the shear wave. Longitudinal waves are different. The longitudinal wave vibrates along the propagation direction of the wave, and the fence or similar obstacle will not prevent the wave propagation in either direction. In terms of this property, the vibration of longitudinal wave is axisymmetric to the propagation direction of wave, and the vibration of transverse wave is not axisymmetric to the propagation direction of wave. The above characteristics of shear wave are its polarization.

According to the polarization characteristics, light waves can be divided into natural light, partially polarized light, linearly polarized light, circularly polarized light and elliptically polarized light. In the process of light propagation, there is only one kind of vibration, and its vibration direction is always kept in the same plane. This kind of light is called linearly polarized light. Light wave contains transverse vibration in all possible directions, but the amplitudes in different directions are different. The amplitudes in two mutually perpendicular directions have maximum and minimum values. This kind of light is called partially polarized light. In the process of light propagation, the electric vector of each point in space rotates with the light as the axis, and the endpoint of the electric vector traces an elliptical trajectory. This kind of light is called elliptically polarized light.