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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 ?
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- 4.5 cm downward
- 1 cm downward
- 2 cm upward
- 1 cm upward
Correct Option: D
In the later case microscope will be focussed for O'. So, it is required to be lifted by distance OO'. OO' = real depth of O – apparent depth of O. 
| = 3 - |  | μ = |  | |||
| 1.5 | apparent depth |
| = 3 | | μ = | | = | = 1 cm | |||
| 1.5 | 1.5 |
