Optics 6 Question 12

14. In a Young’s double slit experiment, slits are separated by 0.5 mm and the screen is placed 150cm away. A beam of light consisting of two wavelengths, 650nm and 520nm, is used to obtain interference fringes on the screen. The least distance from the common central maximum to the point where the bright fringes due to both the wavelengths coincide, is

(a) 7.8mm

(b) 9.75mm

(c) 15.6mm

(d) 1.56mm

(2017 Main)

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Solution:

  1. Let nth bright fringes coincides, then

yn1=yn2 n1λ1Dd=n2λ2Ddn1n2=λ1λ2=520650=45

Hence, distance from the central maxima is

y=n1λ1Dd=4×650×109×1.50.5×103=7.8mm



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