Question

If a ray of light travels from water (μ_w) to air (μ_a). Find the critical angle?

Answer 1

When angle of incidence is equal to critical angle, angle of refraction is 90 degree.
Apply Snell's law at interface

${\mu }_w$ sin $i_c$ = ${\mu }_a$ sin ${90}^{\circ }$ , where sin ${90}^{\circ }$ = 1.

$i_c$ = ${\sin }^{-1}$ $\frac{{\mu }_a}{{\mu }_w}$