A microscope is focused on a mark on a piece of paper and then a slab of glass of thickness `3cm` and refractive index `1.5` is placed over the mark. How should the microscope be moved to get the mark in focus again ?

Options

A. `2 cm` upward
B. `1 cm` upward
C. `4.5 cm` upward
D. 1 cm downward

Answer

B

Explanation

Here, real depth of mark, `x = 3 cm`
apparent depth of mark, `y = ?`
refractive index, `mu = 1.5`
As `mu = (x)/(y) :. y = (x)/(mu) = (3)/(1.5) = 2 cm`
Distance through which mark appears to be raised
`= x -y = 3 - 2 = 1 cm`.
`:.` To get the mark in focus again, distance through which microscope be moved upwards `= 1 cm`