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In Young's experiment the fourth bright fringe produced by light of wavelength 5000 A falls on the fifth bright fringe of an unknown wavelength. The unknown wavelength is

Asked in GUJCET 2019 · Fringe width and fringe position

• (1) 4000 A
• (2) 6000 A
• (3) 5000 A
• (4) 8000 A

Step-by-step solution

Why the other options are wrong

• (2) Follows from inverting the condition, λ₂=(5×5000)/4 -- and even that gives 6250 A. Reaching the same point in five steps instead of four needs the shorter wavelength, not the longer.
• (3) Assumes the two wavelengths are equal. Then the fourth and fifth bright fringes would be one whole fringe apart and could not fall on each other.
• (4) Longer still, and no route from n₁λ₁=n₂λ₂ produces it. Sanity check: the higher order carries the smaller wavelength, so λ₂<5000 A.

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