Light from a denser medium 1 passes to a rarer medium 2. When the angle of incidence is `theta` the partially reflected and refracted rays are mutually perpendicular. The critical angle will be

Explanation

`i+r=90^@impliesr=90^@-i`
`mu_1sini=mu_2sinr`
`mu_1sini=mu_2sin(90^@-i)=mu_2cosi`
`tan i=mu_2/mu_1`
`mu_1sini=mu_2sinr`
When `r=90^@`, `i=C`
`mu_1sinC=mu_2`
`sinC=(mu_2)/(m_1)=tani`
`C=sin^-1(tani)`