Obtain the equation for critical angle.

Explanation

Snell.s law in the product form, equation for critical angle incidence becomes,
`n_(1)sini_(c)=n_(2)sin90^(@)`
`n_(1)sini_(c)=n_(2)(thereforesin90^(@)=1)`
`sini_(c)=(n_(2))/(n_(1))`
Here `n_(1) gt n_(2)`
If the rarer medium is air then is refractive index is I and can be taken as n itself. i.e. `(n_(2) - 1) and (n_(1) = n)`.
`sin i_(c) = (1)/(n) (or) i_(c) = sin^(-1) ((1)/(n))`