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  4. A microscope is focussed on a3 mark on a piece of paper and then a slab of glass of thickness 3 cm and refractive index 1.5 is placed over a mark. How should the microscope be moved to get the mark again in focus?

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Q. A microscope is focussed on a3 mark on a piece of paper and then a slab of glass of thickness 3 cm and refractive index 1.5 is placed over a mark. How should the microscope be moved to get the mark again in focus?

Jharkhand CECEJharkhand CECE 2010Ray Optics and Optical Instruments

Solution:

Apparent depth of mark as seen through a glass slab of thickness $ x $ and refractive index $ \mu $ is
$ \text{Apparent}\,\text{depth}\,\text{=}\,\frac{\text{Real}\,\text{depth}}{\text{Refractive}\,\text{index}} $
or $ x=\frac{x}{\mu }=\frac{3}{1.5}=2\,cm $
As image appears to be raised by 1 cm, therefore, microscope must be moved upwards by 1 cm.