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This experiment looks simple: focus three times, subtract, divide. Students still lose practical and viva marks on it, usually by misreading the vernier, re-focusing the eyepiece halfway through, or not knowing which reading is which. This post walks through the experiment as the NCERT laboratory manual sets it out, with a worked set of sample readings, the error calculation and the questions examiners ask.
Which exams include this experiment?
It is experiment 6 in Section B of the CBSE Physics curriculum for 2026-27. It is also item 15 under Experimental Skills in both the JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus, so you can expect a written question on it as well as the practical. Check the current year's documents for your exam.
What is the principle?
The NCERT Class 12 laboratory manual, Experiment 14, states it directly: a slab's bottom surface, seen from above, appears raised by refraction, and for viewing along the normal, refractive index = real thickness ÷ apparent thickness. The derivation is the same as for a coin in water, which our post on apparent depth and normal shift works through. As a sanity check, the NCERT Ray Optics chapter lists crown glass at 1.52.
How do you find the least count?
The manual's example scale has 20 main scale divisions in 1 cm, so 1 MSD = 0.05 cm, and 50 vernier divisions equal 49 main scale divisions. The least count is 1 MSD − 1 VSD = 0.05 × (1 − 49/50) = 0.001 cm.
A reading is main scale reading + (coinciding vernier division × least count). Check your own instrument; not every microscope has this scale.
Which three readings do you take?
a₁: focus on a mark on paper on the platform.
a₂: place the slab over the mark and raise the microscope until the mark, seen through the glass, is sharp.
a₃: sprinkle a thin layer of lycopodium powder or chalk dust on top of the slab and raise the microscope until the particles are sharp.
The microscope moves up each time, so a₁ < a₂ < a₃. Then:
real thickness = a₃ − a₁ (from the mark to the top)
apparent thickness = a₃ − a₂ (from the raised image to the top)
n = (a₃ − a₁) ÷ (a₃ − a₂)
Different books label the readings differently, so write the two thicknesses from what you focused on rather than copying a formula.
Worked example with sample readings
These are sample numbers for practice, with the least count of 0.001 cm.
Mark: main scale 2.30 cm, vernier division 15, so a₁ = 2.30 + 15 × 0.001 = 2.315 cm.
Mark through slab: 2.95 cm and division 39, so a₂ = 2.989 cm.
Top surface: 4.30 cm and division 35, so a₃ = 4.335 cm.
Real thickness = 4.335 − 2.315 = 2.020 cm. Apparent thickness = 4.335 − 2.989 = 1.346 cm. n = 2.020 ÷ 1.346 = 1.501.
Error. The manual gives Δn = 2n(Δa/b + Δa/c), where b and c are the real and apparent thickness and Δa is the least count. Each thickness is a difference of two readings, which is where the factor 2 comes from.
Δn = 2 × 1.501 × (0.001/2.020 + 0.001/1.346) = 3.002 × (0.000495 + 0.000743) ≈ 0.004.
Result: n = 1.501 ± 0.004. The manual asks you to invert the slab, repeat, take the mean of n and report the larger of the two errors.
Why repeat the readings at all?
Allie and colleagues (1998) used written probes with first-year university science students to study how they thought about the reliability of data, including the need to repeat measurements and what the spread in data means. They found that students' use of these ideas depended on the context, and that their intuitive ideas about improving precision and accuracy were not distinct. The second set with the slab inverted is not a formality: if the two values of n differ by much more than Δn, something went wrong, usually focus or a misread vernier.
What mistakes cost marks?
The manual lists these precautions and sources of error:
Turning the focusing screw back and forth, which introduces backlash. Move it in one direction only.
Changing the eyepiece focus after a₁.
A thick layer of powder, which no longer marks the top surface.
A microscope that is not normal to the slab, or a slab that is not horizontal.
Reading the vernier without a hand lens, so the wrong division is chosen.
What viva questions should you prepare?
The manual's self-assessment asks two good ones.
Would a colourless slab be visible in a liquid of the same refractive index? No. With no change in refractive index at the surface, light passes straight through without refraction or reflection there, so the edges cannot be seen.
Three slabs of the same size, one hollow and filled with water, one of crown glass and one of flint glass: in which does a mark at the bottom look most raised? The shift is t(1 − 1/n), which grows with n, so the flint glass slab.
Be ready to explain why the formula needs near-normal viewing: it uses the small-angle step sin ≈ tan.
If you want to see how the image rises as n changes, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. For the theory behind each step, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing.
A practice routine before the practical
Read five vernier positions from a photo or diagram and check them with a partner.
Rework the sample readings above without looking, including Δn.
Write the five precautions from memory.
Answer the two viva questions aloud in under a minute each.
Before your lab slot, write the table headings for a₁, a₂ and a₃ in your notebook so you only have to fill in numbers.
