Question

Coherent light with wavelength 600 nm passes through two very narrow slits and the interference pattern is observed on a screen 3 m from the slits.The first order bright fringe is at 4.84 mm from the centre of the central bright fringe.For what wavelength of light will the first order dark fringe be observed at the same point on the screen?

Answer 1

find distance d of the slits  

y = L*λ/d  

0.00484 = 3*610*10^-9/d  

d = 0.000378099 m  

λ of first dark fringe at y = 0.00484 m is found by  

0.00484 = 1/2*3*λ/0.000378099  

λ = 1220*10^-9 m  

= 1.22*10^-6 m  

= 1.22 μm  

Hope it helps

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