A tank is filled with water to a height of 12.5 cm. The apparent depth of a needle lying at the bottom of the tank is measured by a microscope to be 2.54 cm. What is the refractive index of water ? If water is replaced by a liquid of refractive index 1.63 up to the same height, by what distance would the microscope have to be moved to focus on the needle again ?

Explanation

`mu=("Real depth")/("Apparent depth")`
`mu=(12.5)/(9.4)=1.33`
Again for a liquid
`mu=1.63`
`"Apparent depth "=("Real Depth")/(mu)`
`=(12.5)/(1.63)=7.668`
Distance by which image becomes raised `=9.4-7.67=1.73=1.7cm` the microscope will be moved up by 1.7 cm to focus on the needle again.