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In Young's double slit experiment, the slits are separated by 0.54 mm and the screen is placed 1.8 m away. The distance between central bright fringe and sixth bright fringe is measured to be 1.2 cm. Determine the wavelength of light used in the experiment.

Asked in GUJCET 2025 · Fringe width and fringe position

• (1) 5000 A
• (2) 600 nm
• (3) 8000 nm
• (4) 800 nm

Step-by-step solution

Why the other options are wrong

• (1) No slip with these numbers gives 5000 A; it is the wavelength students reach for out of habit. Sanity check: λ=(y₆d)/(6D)=(1.2×10⁻²×0.54×10⁻³)/(6×1.8)=6×10⁻⁷ m.
• (3) 8000 nm is 8 μm, far into the infrared -- invisible, so it could not produce fringes anyone could measure with a scale. It is also ten times option (D).
• (4) 800 nm would need the sixth fringe at 1.6 cm; the measured distance is 1.2 cm, which gives 600 nm.

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