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Question:

Two slits are separated by a distance of 0.5 mm and illuminated with light of wavelength λ=6000 Å. If the screen is placed at 2.5 m from the slits. The distance of the third bright fringe from the centre will be

3 mm

6 mm

1.5 mm

9 mm

Solution:

Distance of nth bright fringe from the centre, xn = nDλ/d
So, x3 = 3 × 2.5 × 6000 × 10-10 / 0.5 × 10-3 = 9 × 10-3 m = 9 mm.