(d) $\beta = \frac{\lambda D}{d} \Rightarrow \left( 4 \times 10^{- 3} \right) = \frac{4 \times 10^{- 7} \times D}{0.1 \times 10^{- 3}} \Rightarrow D = 1m$
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Ph.D. in Physics with 19+ years of teaching experience.