(b) $E = 3eV = 3 \times 1.6 \times 10^{- 19}$ J $\lambda = \frac{h}{\sqrt{2mE}}$ $= \frac{6.6 \times 10^{- 34}}{\sqrt{2 \times 1.7 \times 10^{- 27} \times 3 \times 1.6 \times 10^{- 19}}} = 1.65 \times 10^{- 11}m$
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Ph.D. in Physics with 19+ years of teaching experience.