Question -
Answer -
Actual depth of the needle in water, h1 = 12.5 cm
Apparent depth of the needle in water, h2 = 9.4 cm
Refractive index of water = μ
The value of μcan be obtained asfollows:
Hence, the refractiveindex of water is about 1.33.
Wateris replaced by a liquid of refractive index, 
The actual depth of the needle remains thesame, but its apparent depth changes. Let y be the newapparent depth of the needle. Hence, we can write the relation:
Hence, the new apparent depth of the needleis 7.67 cm. It is less than h2. Therefore, to focus theneedle again, the microscope should be moved up.
∴Distanceby which the microscope should be moved up = 9.4 − 7.67
=1.73 cm