A light beam passes from medium 1 to medium 2. Show that the emerging beam is parallel to the incident beam.

Explanation

Applying Snell's law at A and B,
`mu_(1)sini_(1)=mu_(2)sini_(2)`

or `mu_(1)/mu_(2)=(sini_(2))/(sini_(1))`
Similarly `mu_(2) sini_(2)=mu_(1)i_(3)`
`therefore` `mu_(1)/mu_(2)=(sini_(2))/(sini_(3))`
From Eqs. (i) and (ii),
`i_(3)=i_(1)`
i.e, the emrgent ray is parallel to incident ray.