In the above example, find the angle of incidence at face `AB`, so that emergent ray grazes along the face `AC'`.

Explanation

Calculation of angle of incidence at face AB.

At face AC, let the angle of incidence be `r_(2)`, For grazing ray `e=90^(@)`
`impliesmu=1/(sinr_(2))=r_(2)=sin^(-1)(1/(sqrt(2)))=45^(@)`
Let angle of refraction at face AB be `r_(1)` Now `r_(1)+r_(2)=A`
`:.R_(1)=A-r_(2)=60^(@)-45^(@)=15^(@)`
Let anle of incidence at this face be i
`mu=(sini)/(sinr_(1))`
`impliessqrt(2)=(sini)/(sin 15^(@))`
`:.i=sin^(-1)(sqrt(2).sin 15^(@))`