b
(b) Angular width \(\beta \)\( = \frac{{2\lambda }}{d}\)
\( \Rightarrow \frac{{{\beta _1}}}{{{\beta _2}}} = \frac{{{\lambda _1}}}{{{\lambda _2}}} \Rightarrow \frac{\beta }{{\frac{{70}}{{100}}\beta }} = \frac{{6000}}{{{\lambda _2}}} \Rightarrow {\lambda _2} = 4200\,{Å}\)