Wave Optics

Concept:Brewster's law states that light reflected from a boundary is completely polarized when the reflected and refracted rays are perpendicular to each other.Explanation:The reflected ray $CD$ is polarized when the angle of incidence at the final boundary (water to prism) satisfies Brewster's law.At that boundary, the initial medium is water ($n_w = \frac{4}{3}$) and the final medium is prism ($n_p = \frac{4}{\sqrt{3}}$).Using Brewster's law: $\tan \theta = \frac{n_p}{n_w} = \frac{4/\sqrt{3}}{4/3} = \frac{3}{\sqrt{3}} = \sqrt{3}$.Since $\tan 60^\circ = \sqrt{3}$, the angle of incidence at the bottom water–prism interface is $\theta = 60^\circ$.Now apply Snell's law sequentially through the layers: water → glass → water → prism.At the top water–glass boundary: $n_w \sin i = n_g \sin r_g$.At the glass–water boundary: $n_g \sin r_g = n_w \sin r_w$.At the bottom water–prism boundary: $n_w \sin r_w = n_p \sin r_p$.Combining all gives $n_w \sin i = n_w \sin \theta$, so $\sin i = \sin \theta$.Since $\theta = 60^\circ$, we obtain $i = 60^\circ$.Answer:$60^\circ$ (i.e., 60 degrees)