Q4. Assertion: only a thin layer of powder should be sprinkled on the top face of the slab. Reason: a thick layer does not mark the true top face, so the reading a3 is wrong. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion (Laboratory Manual, Experiment 14, page 108).
Q5. The readings are a1 = 2.300 cm, a2 = 3.100 cm and a3 = 4.700 cm. Find the refractive index of the slab.
Real thickness = 4.700 − 2.300 = 2.400 cm; apparent thickness = 4.700 − 3.100 = 1.600 cm; n = 2.400/1.600 = 1.5.
Q6. A mark lies under a slab 4.0 cm thick of refractive index 1.6. By how much does it seem raised when viewed from above?
t(1 − 1/n) = 4.0 × (1 − 1/1.6) = 1.5 cm (reasoned from NCERT page 229).
Q7. What is backlash error in a travelling microscope, and how is it avoided?
When the focusing screw reverses direction, it turns a little before the microscope moves, so the reading slips. Turn the screw in one direction only (Laboratory Manual, Experiment 14, page 108).
Q8. NCERT Exercise 9.3: a needle under 12.5 cm of water appears 9.4 cm deep through a microscope. Find n. If a liquid of n = 1.63 replaces the water, how far must the microscope move to refocus?
n = 12.5/9.4 ≈ 1.33; the new apparent depth is 12.5/1.63 ≈ 7.67 cm, so the microscope is raised by about 1.7 cm (NCERT Exercise 9.3, page 249; answer page 346).