Which is easier to travel through, air or water?

People usually travel faster through air. So does light!

Definition: Refractive Index
The refractive index (symbol n) of a material is the ratio of the speed of light in a vacuum to its speed in the material and gives an indication of how difficult it is for light to get through the material.

n = c / v
where
n = refractive index (no unit)
c = speed of light in a vacuum (3 × 10⁸m s⁻¹)
v = speed of light in a given medium (m s⁻¹)

If the refractive index n increases, the speed of light in the material v must decrease. Light therefore travels slowly through materials of high n.


Light travels slower in any material than it does in a vacuum, so all values for n are greater than 1.


Refractive indices of some materials.

Quiz:
1. Describe what is meant by the refractive index of a medium.

2. When light travels from air to a medium, its speed is decreased by 25%. What is the refractive index of the medium ?
(ans : Q2 = 1.33)

3. Light travels from a medium with n = 1,63 into a medium of n = 1,42.
(a) What happens to the speed of the light? Does it increase, decrease, or remain the same?
(b) Does the light bend towards the normal, away from the normal, or not at all?