A transverse wave has a plane of vibration, or a plane that indicates a vibration direction. (A longitudinal wave does not have this.) There can exist many directions of vibration in one plane of vibration. As a result, there are many waves with different directions of vibration, even if they share the same wavelength and propagation path.

Polarization occurs when the directions of vibration concentrate in a certain direction.

Polarization proves that light is a transverse wave.

You can confirm polarization with a simple test like the one in the diagram below. The vibration of light from a light source can be multidirectional. If you guide light through equally spaced vertical slits, only light that vibrates vertically can pass through. (The state of polarized light in a certain linear direction is called "linear polarization.") If light, after passing through vertical slits, is transmitted into horizontal slits, then you see darkness on the other side, as no light can pass through. A polarizing filter acts like a combination of these slits. Polarizing filters are commonly used with single-lens reflex (SLR) cameras.