Refractive Index

The refractive index (n) of a medium measures how much light slows down when it enters that medium. A higher refractive index means light travels more slowly in that medium.

Equations

n = speed of light in vacuum / speed of light in the medium

n = sin(angle of incidence) / sin(angle of refraction) (Snell’s law, for light going from a vacuum or air into the medium)

n = 1 / sin(critical angle)

Typical values

• Air / vacuum: n = 1.0
• Water: n = 1.33
• Glass: n = 1.5 (approximately)
• Diamond: n = 2.42

Key points

• The refractive index of a medium is always greater than or equal to 1.
• A higher refractive index means the light bends more when entering the medium.
• Total internal reflection only occurs when light travels from a medium with a higher refractive index to one with a lower refractive index.

In exam questions (Extended)

Use Snell’s law to calculate angles or the refractive index. Calculate the critical angle using sin(c) = 1/n. Show all substitution and rearrangement steps.