Light and optics

The fastest thing there is.

A constant speed

Light travels at about 300,000 kilometres per second in a vacuum, and that speed is the same for every observer however they are moving. Special relativity is what follows from taking that seriously.

Visible light is a narrow band of the electromagnetic spectrum.
Radio, infrared, ultraviolet, X-rays and gamma rays are the same phenomenon at other frequencies.

Bending and splitting

Light slows in glass or water and changes direction, which is refraction and how lenses work. Different colours bend by different amounts, which is why a prism produces a spectrum.