\(n_{d} \sin i_{c}=n_{r} \sin 90^{\circ}\)

\(\sin i_{ C }=\frac{ n _{ r }}{ n _{ d }}=\frac{ V _{ d }}{ V _{ I }}\)

\(\sin i_{c}=\frac{1.5 \times 10^{8}}{2 \times 10^{8}}=\frac{1.5}{2}\)

\(\sin i_{c}=\frac{3}{4}\)

\(\tan i_{c}=\frac{3}{\sqrt{4^{2}-3^{2}}} \Rightarrow \frac{3}{\sqrt{7}}\)

\(i_{c}=\tan ^{-1}\left(\frac{3}{\sqrt{7}}\right)\)