White light can be split up to form a spectrum using a prism. This is a block of glass with a triangular cross section.
The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most - causing the coloured light to spread out to form a spectrum. This is called dispersion.