A ray of light falls on a glass plate of refractive index `mu=1.5`.
What is the angle of incidence of the ray if the angle between the reflected and
refracted rays is `90^@`?

Explanation

If i and r are the angle of incident and angle of refraction respectively then
`i+r=90^(@)`
`:.r=90^(@)-i`

By Snell.s law `(sin i)/(sin r)=mu`
or `(sin i)/(sin (90^(@)-i))=mu`
or `(sin i)/(cos i )=mu`
or `tan i=mu`
`:.i=tan^(-1)(mu)=tan^(-1)(1.5)`