Find the angle of incidence at face AB so that the emergent ray grazes along the face AC.
from snell's law we know that
For the emergent ray to graze at the face AC, the angle of refraction should be 90°.
So, applying snell's law at face AC, we get `(sini_(AC)/sinr_(AC) )= μ_(21)`
`⇒ (sin i_(AC)/sin 90^@)=1/sqrt2 or i_(AC)= 45°.`
From geometry we can see that angle of refraction at face AB is 15°.
So applying snell's law we get `(sin i_(AB)/sin r_(AB))= μ_(12)`
`or sin i_(AB) = sin r_(AB)×μ_(12)`
`or i_(AB) =sin^-1(sqrt2sin15°)`