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Question:
In a Youngs double-slit experiment, the slits are separated by 0.28 mm and the screen is placed 1.4 m away. The distance between the central bright fringe and the fourth bright fringe is measured to be 1.2 cm. Determine the wavelength of light used in the experiment.
Answer:

Distance between the slits, d = 0.28 mm = 0.28 x 10-3 m

Distance between the slits and the screen, D = 1.4 m

Distance between the central fringe and the fourth (n = 4) fringe,

u = 1.2 cm = 1.2 x 10-3 m

In case of a constructive interference, the relation for the distance between the two fringes is as

u = nλD/d   Where, Order of fringes (n) = 4, Wavelength of light=λ

Therefore,

λ=ud/nD

 = 1.2 x 10-3 x 0.28 x 10-3/ 4 x 4.14

= 6 x 10-7

= 600 nm

Hence, the wavelength of the light is 600 nm.

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