a
(a)As \(\beta = \frac{{D\lambda }}{d} \Rightarrow \frac{{{\beta _1}}}{{{\beta _2}}} = \left( {\frac{{{D_1}}}{{{D_2}}}} \right)\;\left( {\frac{{{\lambda _1}}}{{{\lambda _2}}}} \right)\;\left( {\frac{{{d_2}}}{{{d_1}}}} \right)\) \( \Rightarrow 1 = \left( {\frac{{{D_1}}}{{{D_2}}}} \right) \times \left( {\frac{1}{2}} \right) \times \left( {\frac{1}{2}} \right) \Rightarrow \frac{{{D_1}}}{{{D_2}}} = \frac{4}{1}\)