Spectrum
A natural example of a spectrum is a rainbow. The word spectrum was first used by scientists studying optics. They used the word to describe the rainbow of colors in visible light when separated using a prism.  The spectrum seen when light passes through a prism is an example of the dispersion of light.  The material from which the prism is made has a different  refractive index n than air.  Usually, nprism is greater than nair, and nair is taken to be approximately one. This implies that light travels a little slower in the material of the prism than in the space surrounding it.  The angle of refraction can be determined from the angle of incidence and the refractive indexes using Snell's law.