Wave Optics - Result Question 20
20. In Young’s experiment, two coherent sources are placed $0.90 mm$ apart and fringe are observed one metre away. If it produces second dark fringe at a distance of $1 mm$ from central fringe, the wavelength of monochromatic light used would be
(a) $60 \times 10^{-4} cm$
(b) $10 \times 10^{-4} cm$
(c) $10 \times 10^{-5} cm$
(d) $6 \times 10^{-5} cm$
[1991, 1992]
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Answer:
Correct Answer: 20. (d)
Solution:
- (d) For dark fringe
$ \begin{aligned} & x=(2 n-1) \frac{\lambda D}{2 d} \\ & \therefore \lambda=\frac{2 x d}{(2 n-1) D}=\frac{2 \times 10^{-3} \times 0.9 \times 10^{-3}}{(2 \times 2-1) \times 1} \end{aligned} $
$\lambda=0.6 \times 10^{-6} m=6 \times 10^{-5} cm$
