Question:

A fringe width of 6 mm was produced for two slits separated by 1 mm apart. The screen is placed 10 m away. The wavelength of light used is 'x' nm. The value of 'x' to the nearest integer is ____________.

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Use the fringe width formula \(\beta = \frac{\lambda D}{d}\) and convert all lengths to metres.
Updated On: Nov 4, 2026
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Correct Answer: 600

Solution and Explanation

Fringe width in Young's double-slit experiment: \(\beta = \frac{\lambda D}{d}\).
Given \(\beta = 6\ \mathrm{mm} = 6\times10^{-3}\ \mathrm{m}\), \(d = 1\ \mathrm{mm} = 10^{-3}\ \mathrm{m}\), \(D = 10\ \mathrm{m}\).
\(\lambda = \frac{\beta d}{D} = \frac{6\times10^{-3}\times 10^{-3}}{10} = 6\times10^{-7}\ \mathrm{m}\).
\(\lambda = 600\ \mathrm{nm}\), so \(x = 600\).
Answer: 600
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