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  • In a Fresnel's Biprism experiment Interference Fringes are observed with a biprism of refracting angle 2° and refractive index 1.5 on a screen which is  100m away from the so

In a Fresnel's Biprism experiment Interference Fringes are observed with a biprism of refracting angle 2° and refractive index 1.5 on a screen which is  100m away from the source. If the distance between the source and the biprism is 20m and the Fringes width is 0.10 mm what is the wavelength of light ?

Option: 1

69.7 nm


Option: 2

6.97 mm


Option: 3

697 nm


Option: 4

None of these 


Answers (1)

best_answer

first of all find deviation ,  \delta=(\mu-1)A

we know, 
where a is distance between source and biprism. e.g., a = 20m
so,  d = 2 \times \frac{ \pi }{180} \times 20 = \frac{2\pi}{9} \ m

now, use formula of fringe width, 

here, 
D = 100m , d = \frac{2\pi}{9} \ m


so, 0.1 \times 10^{-3} =\lambda \times \left( \frac{100}{\frac{2\pi}{9}} \right) d 
=> 10^{-4} \times \frac{2\pi}{9} \times 10^{-2} = \lambda
=> λ = 697 nm

The correct option is 3.

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mansi

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