Ray Optics and Optical Instruments - Result Question 69
73. A microscope is focussed on a mark on a piece of paper and then a slab of glass of thickness 3 $cm$ and refractive index 1.5 is placed over the mark. How should the microscope be moved to get the mark in focus again?
[2006]
(a) $4.5 cm$ downward (b)
(b) $1 cm$ downward
(c) $2 cm$ upward
(d) $1 cm$ upward
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Answer:
Correct Answer: 73. (d)
Solution:
- (d) In the later case microscope will be focussed for $O^{\prime}$. So, it is required to be lifted by distance $O O^{\prime}$. $O O^{\prime}=$ real depth of $O-$ apparent depth of $O$.
$=3-\frac{3}{1.5} \quad[\mu=\frac{\text{ real depth }}{\text{ apparent depth }}]$
$=3[\frac{1.5-1}{1.5}]=\frac{3 \times .5}{1.5}=1 cm$
