\(1 = 1 + 1 + 2\cos \theta \)
\(\cos \theta = - \frac{1}{2}\)
\(\theta = 120^\circ \)
\(n_d^2 = n_1^2 + n_2^2 - 2{n_1}{n_2}\cos \theta \)\( = 1 + 1 - 2\cos (120^\circ )\)
\(n_d^2 = 2 - 2( - 1/2) = 2 + 1 = 3\) \( \Rightarrow \,\,{n_d} = \sqrt 3 \)