Place a mark on a table; focus microscope on the mark (reading R1). Place glass slab on the mark; refocus on the apparent image (reading R2). Sprinkle chalk dust on top surface; focus on dust (reading R3). n = (R3 − R1)/(R3 − R2). The slab causes apparent shift t·(1 − 1/n).
-- NCERT Physics Lab Manual Class 12, Part 10, p. 106Experiment 15 Refractive Index Glass Slab
8 MCQs9-step worked example
Source: NCERT Experimental SkillsOfficial key: NTA-verifiedLast updated: 27 Sep 2026
Experiment 15 Refractive Index Glass Slab, explained for NEET
The trap that costs marks here is a parallax check made against the wrong feature. In a travelling microscope you are not aligning two pins — you are bringing the cross-wire and the image of the mark into the same plane. Students declare focus when the image merely looks sharp, then move the eye across the eyepiece and the cross-wire slides over the image. That slide is parallax, and every reading taken with it is displaced. Match feature to feature: cross-wire on the mark itself, not on its blurred halo, and confirm by moving the eye laterally until the two move together.
The measurement itself is the apparent-depth effect. Light from a mark under a glass slab bends away from the normal on leaving the glass, so the mark appears raised. Real depth divided by apparent depth gives the refractive index. Three vertical settings of the microscope supply both depths, as set out in the NCERT Physics Lab Manual Class 12, Part 10, pages 105–106: focus on the ink mark on the paper (R₁), place the slab over it and focus on the image of the same mark (R₂), then sprinkle fine powder on the top face of the slab and focus on the powder (R₃).
Then real depth = R₃ − R₁ (the slab thickness) and apparent depth = R₃ − R₂. Their ratio is the refractive index. The difference R₂ − R₁ is only the apparent shift — it is not the apparent depth, and dividing by it is the single most common arithmetic error on this experiment.
For NEET the questions are short: predict the direction of the shift, pick the right pair of subtractions, or read a procedural step. The powder is there for one reason — the bare top surface of the slab has nothing for the microscope to focus on.
Watch-out: refocus after each change of setting, and re-run the eye-movement parallax check every time before you read the vernier.
Can you answer these Experiment 15 Refractive Index Glass Slab MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
In this experiment the vernier scale of the travelling microscope is read in order to record, at each setting:
Show answer and why every option is right or wrong
Answer: C. Option C is correct: the microscope is raised or lowered until a feature is in focus, and its vertical scale position is what is recorded; differences between three such positions give both depths (NCERT Physics Lab Manual Class 12, Part 10, page 106).
Why A is wrong: A is wrong because the whole measurement is vertical; no horizontal separation enters the apparent-depth relation.
Why B is wrong: B is wrong because no angle is measured in this experiment — angles of incidence and deviation belong to the prism experiment, not to the glass-slab apparent-depth method.
Why D is wrong: D is wrong because the slab thickness is itself obtained as a difference of two focus positions, R₃ − R₁, not read off in one step.
MCQ 2Easy RecallPractice
A mark on paper viewed vertically downwards through a glass slab appears:
Show answer and why every option is right or wrong
Answer: A. Option A is correct: light leaving the denser glass bends away from the normal, so the mark is seen nearer the top surface and its apparent depth is smaller than the real depth (NCERT Physics Lab Manual Class 12, Part 10, page 105).
Why B is wrong: B is wrong because it reverses the effect; if apparent depth exceeded real depth the ratio real/apparent would be less than 1 and no glass would give a refractive index below unity.
Why C is wrong: C is wrong because parallel faces remove the net deviation of the emergent ray's direction, not the shift of the image along the line of sight.
Why D is wrong: D is wrong because lateral displacement belongs to oblique incidence, and in any case it is far smaller than the slab thickness; here the view is vertical and the shift is along the depth.
MCQ 3Easy RecallPractice
Fine powder such as lycopodium or chalk dust is sprinkled on the upper surface of the slab in order to:
Show answer and why every option is right or wrong
Answer: D. Option D is correct: a clean glass surface offers nothing sharp to focus on, so a dusting of powder supplies the feature whose focus position becomes R₃ (NCERT Physics Lab Manual Class 12, Part 10, page 106).
Why A is wrong: A is wrong because glare control is not the purpose; the reading taken with the powder is a focus position, and a powdered surface in fact scatters more light back, not less.
Why B is wrong: B is wrong because the mark is on the paper beneath the slab and is not touched by powder sprinkled on top.
Why C is wrong: C is wrong because the refractive index of the medium above the slab plays no part in the real-depth/apparent-depth ratio being measured.
MCQ 4Direct ApplicationPractice
A travelling microscope is focused on an ink mark at 4.105 cm, then on the image of the same mark through a glass slab at 4.605 cm, and finally on powder on the top face of the slab at 5.605 cm. The refractive index of the glass is:
Show answer and why every option is right or wrong
Answer: B. Option B is correct: real depth = 5.605 − 4.105 = 1.500 cm, apparent depth = 5.605 − 4.605 = 1.000 cm, so n = 1.500/1.000 = 1.50 (NCERT Physics Lab Manual Class 12, Part 10, page 107).
Why A is wrong: A is wrong because it inverts the ratio, dividing apparent depth by real depth (1.000/1.500); refractive index of glass relative to air is greater than 1.
Why C is wrong: C is wrong because it divides the apparent depth by the apparent shift (1.000/0.500) — the shift R₂ − R₁ is not a depth and belongs in neither position of the ratio.
Why D is wrong: D is wrong because it uses the apparent shift as the denominator (1.500/0.500); the apparent depth is measured down from the top surface, i.e. R₃ − R₂.
MCQ 5Direct ApplicationPractice
A glass slab of refractive index 1.50 and thickness 6.00 cm is laid over a mark on paper. Seen vertically from above, the mark appears raised by:
Show answer and why every option is right or wrong
Answer: D. Option D is correct: apparent depth = 6.00/1.50 = 4.00 cm, so the mark rises by 6.00 − 4.00 = 2.00 cm (NCERT Physics Lab Manual Class 12, Part 10, page 105).
Why A is wrong: A is wrong because it is neither the apparent depth nor the difference; no combination of 6.00 cm and 1.50 produces it.
Why B is wrong: B is wrong because it halves the thickness, which would require a refractive index of 2.00, not 1.50.
Why C is wrong: C is wrong because 4.00 cm is the apparent depth itself, not the rise; the question asks for the difference between real and apparent depth.
MCQ 6Direct ApplicationPractice
While focusing on the image of the mark through the slab, a student moves her eye across the eyepiece and sees the cross-wire slide over the image. The correct conclusion and action are:
Show answer and why every option is right or wrong
Answer: A. Option A is correct: relative motion under lateral eye movement is exactly the parallax test, and it is removed by refocusing until cross-wire and image move as one (the focusing steps are in the NCERT Physics Lab Manual Class 12, Part 10, page 106).
Why B is wrong: B is wrong because slab thickness does not produce relative motion between cross-wire and image; a thick slab simply moves the image to a different focus position.
Why C is wrong: C is wrong because an instrument offset shifts every reading equally and cannot make two features in the same field move relative to each other.
Why D is wrong: D is wrong because resolution limits how finely a position can be stated, while parallax displaces the setting itself — a finer scale would only record the displaced value more precisely.
MCQ 7CalculationPractice
A student's three readings give a real depth of 1.50 cm and an apparent depth of 1.00 cm. She then repeats the whole experiment with a slab of the same glass but twice the thickness. Compared with the first run, the new result is:
Show answer and why every option is right or wrong
Answer: C. Option C is correct: real depth 3.00 cm and apparent depth 2.00 cm keep the ratio at 1.50, while the rise, being real minus apparent, scales with thickness from 0.50 cm to 1.00 cm (NCERT Physics Lab Manual Class 12, Part 10, page 107).
Why A is wrong: A is wrong on both counts: the rise is a difference of depths and grows with thickness, and the refractive index is a property of the glass, not of how much of it is used.
Why B is wrong: B is wrong because it gets the rise right but treats the refractive index as thickness-dependent; doubling both depths leaves their ratio at 1.50.
Why D is wrong: D is wrong because it holds the rise fixed; doubling both depths doubles their difference, so the mark rises twice as far.
MCQ 8Concept TrapPractice
With R₁ the focus on the bare mark, R₂ on its image through the slab and R₃ on powder on the top face, one student computes (R₃ − R₁)/(R₃ − R₂) and another computes (R₃ − R₁)/(R₂ − R₁). Which expression gives the refractive index, and on what grounds?
Show answer and why every option is right or wrong
Answer: B. Option B is correct: the refractive index is real depth over apparent depth, and apparent depth is measured from the top face down to the image, that is R₃ − R₂ (NCERT Physics Lab Manual Class 12, Part 10, page 107).
Why A is wrong: A is wrong because R₂ − R₁ is measured from the mark up to its image — the amount the mark appears to rise — not from the top surface down to the image.
Why C is wrong: C is wrong because the two differences are equal only if the apparent depth happens to equal the shift, which would fix the refractive index at exactly 2; parallel faces do not force that.
Why D is wrong: D is wrong because three focus settings already supply both depths; inverting the slab adds nothing to a slab of uniform material.
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How do you solve a Experiment 15 Refractive Index Glass Slab question? A worked example
- 1
Given.
Travelling-microscope readings, each recorded to the 0.001 cm resolution of its vernier: focus on ink mark on paper, R₁ = 2.512 cm; slab placed over the mark, focus on the image of the mark, R₂ = 3.010 cm; powder sprinkled on the top face, focus on the powder, R₃ = 4.007 cm.
- 2
Required.
The refractive index of the glass, and the distance by which the mark appears raised.
- 3
Concept.
A mark viewed normally through a slab of denser material appears raised. Real depth divided by apparent depth gives the refractive index. Real depth is the distance from the top face down to the mark; apparent depth is the distance from the top face down to the image.
- 4
Formula.
n = real depth / apparent depth = (R₃ − R₁)/(R₃ − R₂); apparent rise = R₂ − R₁.
- 5
Substitution.
Real depth = 4.007 − 2.512 cm. Apparent depth = 4.007 − 3.010 cm. Rise = 3.010 − 2.512 cm.
- 6
Calculation.
Real depth = 1.495 cm; apparent depth = 0.997 cm; n = 1.495/0.997 = 1.4995. Rise = 0.498 cm. The 1 in "real depth divided by apparent depth" and the subtraction itself involve no measured constants, so nothing here except the three readings limits the precision; each reading carries four significant figures, and the quotient is reported to three.
- 7
Final answer.
n = 1.50 (dimensionless); the mark appears raised by 0.498 cm.
- 8
Common trap.
Declaring focus as soon as the image looks sharp. Move the eye across the eyepiece: if the cross-wire drifts over the image of the mark, the two are in different planes and the reading is displaced. Refocus until cross-wire and image move together, then read the scale.
- 9
Similar NEET-style question.
A travelling microscope focused on a mark reads 1.820 cm; with a glass slab over the mark the image focuses at 2.290 cm and powder on the top face at 3.230 cm. Find the refractive index of the glass and the apparent rise of the mark. (Real depth 1.410 cm, apparent depth 0.940 cm, n = 1.50, rise 0.470 cm.)
What to remember before solving Experiment 15 Refractive Index Glass Slab questions
Where do students lose marks on Experiment 15 Refractive Index Glass Slab?
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
2 items — click to collapse
Category: Graph Interpretation
Aligning the eye against the wrong reference point — e.g. against the back of an image pin instead of the tip — produces a parallax error even though the student thinks parallax was eliminated.
When it triggers
Optical bench / travelling-microscope reading where the eye reference is a different feature than the object reference.
How to avoid
Match the parallax-check reference: tip-to-tip, scratch-to-scratch, mark-to-mark — not tip-to-back.
Root cause: concept gap
Correction
Move the eye laterally; if pointer and reflection move relative to each other, there is parallax. Adjust until they move together.
More in Experimental Skills: 4 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
Experiment 15 Refractive Index Glass Slab questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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