JEE Main · 2020 · 09 Jan 2020 · Shift 2 · Wave Optics

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In a Young's double slit experiment 15 fringes are observed on a small portion of the screen when light of wavelength 500 nm is used. Ten fringes are observed on the same section of the screen when another light source of wavelength

λ

is used. Then the value of

λ

is (in nm) ____.

Answer: 750

Solution

The fringe width is

β = \frac{λ D}{d}

, so a fixed stretch of screen holds a number of fringes inversely proportional to

λ

. Fifteen fringes of the

500 nm

light span the same width as ten fringes of the unknown light, hence

15 \times 500 = 10 \times λ

. Solving gives

λ = \frac{15 \times 500}{10} = 750 nm

JEE Main · 2020 · 09 Jan 2020 · Shift 2 · Integer answer

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