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NEET PHYSICSEasy

A microscope is focused on a mark on a piece of paper and then a slab of glass of thickness 3 cm3 \text{ cm} and a refractive index 1.51.5 is placed over the mark. How should the microscope be moved to get the mark in focus again?

A

1 cm1 \text{ cm} upward

B

4.5 cm4.5 \text{ cm} downward

C

1 cm1 \text{ cm} downward

D

2 cm2 \text{ cm} upward

Step-by-Step Solution

  1. Identify the Phenomenon: When a glass slab of thickness tt and refractive index μ\mu is placed over an object, the object appears raised due to refraction. This is known as the normal shift.
  2. Formula for Normal Shift: The shift Δt\Delta t is given by: Δt=t(1−1μ)\Delta t = t \left( 1 - \frac{1}{\mu} \right)
  3. Calculation: Given t=3 cmt = 3 \text{ cm} and μ=1.5\mu = 1.5. Δt=3(1−11.5)\Delta t = 3 \left( 1 - \frac{1}{1.5} \right) Δt=3(1−23)\Delta t = 3 \left( 1 - \frac{2}{3} \right) Δt=3(13)=1 cm\Delta t = 3 \left( \frac{1}{3} \right) = 1 \text{ cm}
  4. Conclusion: The image of the mark is raised by 1 cm1 \text{ cm}. To bring the image back into focus, the microscope must be moved upward by the same distance, i.e., 1 cm1 \text{ cm}.
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