Light is a wave of electromagnetic nature. Such a wave always vibrates normally to its direction of propagation. It need not always be so: for instance, such a wave can just as well vibrate perpendicularly to it or at a different angle. Sunlight, for example, has no special preference for the direction in which it vibrates, and the plane of vibration alternates many times per second. Such light is called unpolarized or natural light. It turns out, however, that there is also light with vibrations remaining in one plane. Such light is called (linearly) polarized.