\(\frac{\mathrm{a} \mathrm{x}}{\mathrm{f}}=3 \lambda\)
(since \(\theta\) is very small so \(\sin \theta \approx \tan \theta \approx \theta=\mathrm{x} / \mathrm{f})\)
or \( \lambda =\frac{a x}{3 f}=\frac{0.3 \times 10^{-3} \times 5 \times 10^{-3}}{3 \times 1} \)
\( = 5 \times {10^{ - 7}}{\rm{m}} = 5000\mathop {\rm{A}}\limits^o \)