the change of direction of a ray of lightas it passes obliquely from one medium

intoanother of different transmission speed

When light travels from a less dense tomore dense medium (light slows down), the ray is refracted toward the normal.

Example: light slows down when it passes from air into water

air

water

n i

r

i > r

When light travels from a more densemedium to a less dense medium (light speeds up),

the ray is refracted away from the normal.

Example: light speeds up when passing from glass into air

air

glass

n i

r i < r

An object’s ability to decrease the speed of light,and therefore cause refraction, is given by its

index of refraction. By definition:

the index of refraction of any transparent substanceis equal to the speed of light in a vacuum

divided bythe speed of light in that substance.

n = c / vn = (3 x 108 m/s) / v

The angles of incidence and refraction are relatedin such a way that n = (sin i)/(sin r), where

i = angle of incidence and r = angle of refraction

whenever light passes from a vacuuminto the substance.

In general, for light passing from medium 1 into medium 2,

n1 sin q1 = n2 sin q2

This relationship is known as Snell’s Law.

n1

n2

q1

q2

Total Internal Reflection mayoccur when light enters a newmedium and speeds up (bendsaway from the normal).

Investigate here.

The maximum angle of incidence in which light may enter air from another substance and not undergo total internal refraction is known as the critical angle, and is related to the index of refraction of the substance by:

sin qc = 1/n

Click here, here, and here to view simulations of Snell’s Law.

View an analytical derivationof the geometricalrelationship here.

Investigate total internal reflection here.