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The de Broglie wavelength of an electron is \(0.4 \times 10^{10} \mathrm{~m}\) when its kinetic energy is \(1.0 \mathrm{keV}\). Its wavelength will be \(1.0 \times 10^{-10} \mathrm{~m}\), when its kinetic energy is
(A) \(0.2 \mathrm{keV}\)
(B) \(0.8 \mathrm{keV}\)
(C) \(0.63 \mathrm{keV}\)
(D) \(0.16 \mathrm{keV}\)

3 Answers

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Ans:(D)
$$
\begin{aligned}
&\lambda=\frac{\mathrm{h}}{\mathrm{p}}, \lambda=\frac{\mathrm{h}}{\sqrt{2 \mathrm{mk}}} \\
&\text { so } \lambda \propto \frac{1}{\sqrt{\mathrm{k}}} \\
&\Rightarrow \frac{0.4 \times 10^{-10}}{1.0 \times 10^{-10}}=\frac{\sqrt{\mathrm{k}}}{\sqrt{1}} \\
&\Rightarrow \quad 0.16 \mathrm{keV}
\end{aligned}
$$
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