Refractive index as a speed ratio

What is Refractive index as a speed ratio?

Refractive index n is defined as the ratio of the speeds of a wave in two different regions: n equals the speed in the first region divided by the speed in the second region. Because a wave slows on entering an optically denser region, n is greater than one for light passing from a vacuum or air into glass or water. It is a pure ratio and has no unit.

This definition is part of the Waves chapter in Cambridge IGCSE Physics.

Concepts related to Refractive index as a speed ratio

• wave speed
• wavelength change
• Critical Angle
• optical density