Question

The critical angle for a light travelling from denser to rarer medium is defined as the angle of incidence at which the light refracts at ${90}^{\circ }$​(angle of refraction). If a ray of light travels from water ( ${\mu }_w$) to air ( ${\mu }_a$), find the critical angle.

• A. sin^-1 (μ_w/ μ_a)
• B. sin^-1 (μ_a/μ_w)
• C. cos^-1 (μ_w/ μ_a)
• D. 90^o

Answer 1

The correct option is B

sin^-1 (μ_a/μ_w)

Apply Snell's law at interface

${\mu }_w$ sin $i_c$ = ${\mu }_a$ sin ${90}^{\circ }$

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