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A travelling microscope is focused on a mark on a table. ...

A travelling microscope is focused on a mark on a table. If a glass block of thickness 18.0cm is placed on the mark, by how many centimeters will the microscope be moved upwards so as to focus the mark once again? [refractive index of glass = 1.5]

A.

27cm

B.

24cm

C.

9cm

D.

6cm

Correct answer is D

Apparent depth = \(\frac{\text{thickness}}{\text{refractive index}}\)

= \(\frac{18}{1.5} = 12cm\)

Upward adjustment = 18 - 12

= 6cm