NVAEasyJEE 2023Interference (Young's Experiment)

JEE Physics 2023 Question with Solution

As shown in the figure, in Young's double slit experiment, a thin plate of thickness t=10μmt = 10 \, \mu\text{m} and refractive index μ=1.2\mu = 1.2 is inserted in front of slit S1S_1. The experiment is conducted in air (μ=1\mu = 1) and uses a monochromatic light of wavelength λ=500nm\lambda = 500 \, \text{nm}. Due to the insertion of the plate, central maxima is shifted by a distance of xβ0x\beta_0. β0\beta_0 is the fringe-width before the insertion of the plate. The value of the xx is _____.

Answer

Correct answer:4

Step-by-step solution

Standard Method

Given: A thin plate of thickness t=10×106mt = 10 \times 10^{-6} \, \text{m}, refractive index μ=1.2\mu = 1.2, and light of wavelength λ=500×109m\lambda = 500 \times 10^{-9} \, \text{m} are used.

Find: The value of xx in the shift xβ0x\beta_0.

When a thin plate is introduced in front of one slit in Young's double slit experiment, the fringe shift is

Δx=t(μ1)λβ0\Delta x = \frac{t(\mu - 1)}{\lambda}\beta_0

Substituting the given values,

Δx=10×106(1.21)5×107β0\Delta x = \frac{10 \times 10^{-6}(1.2 - 1)}{5 \times 10^{-7}}\beta_0 Δx=10×106×0.25×107β0\Delta x = \frac{10 \times 10^{-6} \times 0.2}{5 \times 10^{-7}}\beta_0 Δx=2×1065×107β0=4β0\Delta x = \frac{2 \times 10^{-6}}{5 \times 10^{-7}}\beta_0 = 4\beta_0

Hence, x=4x = 4.

Therefore, the value of xx is 44.

Direct Ratio Method

Given: The shift is written as xβ0x\beta_0.

Find: The numerical value of xx.

Using the fringe shift relation directly,

x=t(μ1)λx = \frac{t(\mu - 1)}{\lambda}

Now,

x=10×106×0.2500×109x = \frac{10 \times 10^{-6} \times 0.2}{500 \times 10^{-9}} x=2×1065×107=4x = \frac{2 \times 10^{-6}}{5 \times 10^{-7}} = 4

Therefore, the value of xx is 44.

Common mistakes

  • Using μ\mu instead of (μ1)(\mu - 1) in the shift formula is incorrect because the extra optical path depends on the excess refractive index over air. Use Δx=t(μ1)λβ0\Delta x = \frac{t(\mu - 1)}{\lambda}\beta_0.

  • Not converting μm\mu\text{m} and nm\text{nm} into SI units can lead to a wrong ratio. Convert 10μm=10×106m10 \, \mu\text{m} = 10 \times 10^{-6} \, \text{m} and 500nm=500×109m500 \, \text{nm} = 500 \times 10^{-9} \, \text{m} before substitution.

  • Confusing the fringe shift with fringe width is incorrect. Here the shift is xβ0x\beta_0, so after finding Δx=4β0\Delta x = 4\beta_0, compare coefficients to get x=4x = 4.

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