Place an object pin and image pin coaxial with the mirror or lens; adjust the image pin until parallax with the image vanishes. Use 1/v − 1/u = 1/f (lens) or 1/v + 1/u = 1/f (mirror) with sign convention. Repeat for several u, plot 1/v vs 1/u — intercept on either axis = 1/f.
-- NCERT Physics Lab Manual Class 12, Part 7, p. 73Experiment 13 Focal Length Parallax
8 MCQs9-step worked example
Source: NCERT Experimental SkillsPYQ coverage: NEET 2026Official key: NTA-verifiedLast updated: 25 Sep 2026
Experiment 13 Focal Length Parallax, explained for NEET
The trap in this experiment is not the mirror formula. It is the parallax check itself. A student slides the image pin until it "looks like" it sits on the image, lines the eye up against the head or the back of the pin rather than its tip, and records the reading. The eye reference and the object reference are then different features, so a real parallax remains while the student believes it has been removed. Every reading taken that way carries the same systematic shift, and repeating the trials does not remove it.
Parallax is relative apparent motion. Two things at the same position show no relative shift when the eye moves sideways across the bench. Two things at different distances always shift apart. So the procedure has two halves, and both must be done: move the eye laterally, and match like reference to like — tip to tip, scratch to scratch. If the tip of the image pin and the tip of the image move together as the eye travels, the pin is at the image. If they separate, it is not, however convincing the view from one fixed eye position.
The NCERT Physics Laboratory Manual sets out the concave-mirror determination on this basis: locate the real image with a pin by removing parallax, read the object and image distances on the bench, and combine them with the mirror relation. A convex lens is handled the same way. A convex mirror cannot be, because its image is virtual — no pin can be put where that image appears to be, so an auxiliary lens or the coincidence method is used instead.
For NEET this surfaces as a short reasoning question, not a long calculation: which reading is unreliable, what the no-parallax condition actually asserts, what a coincidence of object and image implies about the centre of curvature. Watch-out: "I can see them overlap" is never the test. The eye must move.
Can you answer these Experiment 13 Focal Length Parallax 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 an optical-bench experiment, two objects are said to show no parallax with respect to each other when they are:
Show answer and why every option is right or wrong
Answer: C. C is correct — absence of parallax means the two things occupy the same point in space, so a lateral movement of the eye produces no relative displacement between them; this is the location criterion used throughout NCERT Physics Lab Manual Class 12, Part 7, page 69.
Why A is wrong: A is incorrect: apparent size depends on distance from the eye and on the objects' actual sizes, so an image can match a pin in apparent size while lying well in front of or behind it.
Why B is wrong: B is incorrect: brightness is set by the source and the optics, not by position, so a dim image and a bright pin can still coincide exactly.
Why D is wrong: D is incorrect: equal distances on the two sides describe one particular conjugate arrangement, not the no-parallax test, which is about coincidence of the image with the pin whatever the distances happen to be.
MCQ 2Easy RecallPractice
While testing for parallax between the image and the image pin, the eye must be moved:
Show answer and why every option is right or wrong
Answer: A. A is correct — only a sideways displacement of the eye changes the line of sight to two objects at different distances, so only sideways motion reveals their separation, as the parallax procedure in NCERT Physics Lab Manual Class 12, Part 7, page 69 requires.
Why B is wrong: B is incorrect: moving along the line of sight changes only the apparent sizes of the two objects; their apparent positions stay superposed, so a pin that is not at the image still looks as though it is.
Why C is wrong: C is incorrect: a single fixed eye position is exactly the condition under which a misplaced pin appears to coincide with the image, and the movement is the whole point of the test.
Why D is wrong: D is incorrect: a circular motion is only a lateral motion done clumsily, and covering the pin removes the very comparison being made.
MCQ 3Easy RecallPractice
When parallax is being removed between an image pin and the image of an object pin, the two features that must be compared are:
Show answer and why every option is right or wrong
Answer: D. D is correct — the parallax reference must be matched feature for feature, tip to tip, so that coincidence of the two sighted points really means coincidence of position, the discipline stated for the pin method in NCERT Physics Lab Manual Class 12, Part 7, page 69.
Why A is wrong: A is incorrect: tip and base are separated along the pin, so aligning them puts the sighted points at different places and leaves a residual parallax that the student reads as success.
Why B is wrong: B is incorrect for the same reason in reverse: the head sits below the tip, so a head-to-tip match builds a fixed offset into every reading.
Why C is wrong: C is incorrect: consistency does not cure a mismatched reference, and repeating the same wrong pairing in every trial makes the error systematic rather than random, so averaging cannot remove it.
MCQ 4Concept TrapPractice
The focal length of a convex mirror cannot be found by simply placing a search pin where its image lies and removing parallax. The reason is that the image formed by a convex mirror of a real object is:
Show answer and why every option is right or wrong
Answer: B. B is correct — a convex mirror forms a virtual, erect, diminished image behind its reflecting surface, and a physical pin cannot be brought to that location, which is why the convex mirror is reached indirectly in NCERT Physics Lab Manual Class 12, Part 7, page 69 rather than by direct pin location.
Why A is wrong: A is incorrect: a convex mirror never forms a real image of a real object, and an obstructing mount would be a practical nuisance rather than the reason the method fails.
Why C is wrong: C is incorrect: faintness is a matter of illumination and could be cured with a brighter source, whereas the obstacle here is geometric.
Why D is wrong: D is incorrect: a convex mirror behaves the same way for every real object position — always virtual, erect and diminished — so no object placement produces a locatable real image.
MCQ 5Direct ApplicationPractice
For a concave mirror, the object pin is at 30.0 cm from the pole and the image pin, set by removing parallax, is at 20.0 cm from the pole on the same side. The focal length of the mirror is:
Show answer and why every option is right or wrong
Answer: A. A is correct — combining the measured distances through the mirror relation gives 1/f = 1/20.0 + 1/30.0 = 5/60.0, so f = 12.0 cm, the standard u–v computation of the concave-mirror determination in NCERT Physics Lab Manual Class 12, Part 7, page 69.
Why B is wrong: B is incorrect: 10.0 cm is the plain difference of the two distances, 30.0 − 20.0, obtained by subtracting the distances instead of adding their reciprocals.
Why C is wrong: C is incorrect: 25.0 cm is the arithmetic mean of 20.0 cm and 30.0 cm, which is what a student gets by averaging u and v rather than combining their reciprocals.
Why D is wrong: D is incorrect: 50.0 cm is the plain sum u + v, a quantity that has no role in the mirror relation.
MCQ 6Direct ApplicationPractice
A student sets the image pin by sighting the top of the image against the head of the pin rather than against its tip, and does this identically in all six trials. The effect on the reported image distance is:
Show answer and why every option is right or wrong
Answer: D. D is correct — matching a pin head to an image tip fixes a constant offset between the two sighted points, so every reading is displaced the same way; this is the residual-parallax failure that the pin procedure in NCERT Physics Lab Manual Class 12, Part 7, page 69 is designed to prevent.
Why A is wrong: A is incorrect: a consistently repeated wrong reference produces a bias, not scatter, and only random errors shrink on averaging.
Why B is wrong: B is incorrect: the head and the tip are separated on the pin, so the eye's lines of sight to them differ and the judged coincidence occurs at a different bench position.
Why C is wrong: C is incorrect: the mirror relation accepts whatever distances are fed into it, so a corrupted image distance yields a corrupted focal length.
MCQ 7Direct ApplicationPractice
A concave mirror has a radius of curvature of 40.0 cm. On the bench, the object pin coincides with its own real image, with parallax removed, when the pin is at a distance from the pole of:
Show answer and why every option is right or wrong
Answer: C. C is correct — object and image coincide only at the centre of curvature, where the rays strike the mirror normally and retrace their path, so the pin sits at 40.0 cm; this coincidence method is the quick focal-length check described in NCERT Physics Lab Manual Class 12, Part 7, page 69.
Why A is wrong: A is incorrect: 10.0 cm is half the focal length, a position inside the focus where a concave mirror forms a virtual image that no pin can be set against.
Why B is wrong: B is incorrect: 20.0 cm is the focal length itself, where the reflected rays emerge parallel and no image is formed on the bench at all.
Why D is wrong: D is incorrect: 80.0 cm is twice the radius of curvature, and the image formed from there lies between the focus and the centre of curvature, well short of the pin.
MCQ 8CalculationPractice
With a convex lens, the object pin is at 15.0 cm on one side and the image pin, set by removing parallax, is at 30.0 cm on the other side. If the same lens is now used in the arrangement where the object pin and the image pin are equidistant from the lens, that common distance is:
Show answer and why every option is right or wrong
Answer: B. B is correct — the first setting gives 1/f = 1/30.0 + 1/15.0 = 3/30.0, so f = 10.0 cm, and object and image are equidistant only at twice the focal length, giving 20.0 cm on each side, the conjugate arrangement used as a cross-check in NCERT Physics Lab Manual Class 12, Part 7, page 69.
Why A is wrong: A is incorrect: 15.0 cm reuses the original object distance, and that setting is precisely the unequal one, with its image out at 30.0 cm.
Why C is wrong: C is incorrect: 22.5 cm is the mean of 15.0 cm and 30.0 cm, which confuses averaging distances with the reciprocal combination the lens relation demands.
Why D is wrong: D is incorrect: 45.0 cm is the sum of the two given distances, a quantity that does not enter the lens relation at all.
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How do you solve a Experiment 13 Focal Length Parallax question? A worked example
- 1
Given.
A concave mirror is mounted on an optical bench. The object pin stands 30.0 cm from the pole. The image pin is moved until the tip of the pin and the tip of the real image show no relative shift as the eye is moved laterally; it then stands 20.0 cm from the pole, on the same side.
- 2
Required.
The focal length of the mirror, reported to the precision the readings support.
- 3
Concept.
The image pin marks the image position only when parallax has genuinely been removed — tip against tip, eye moving sideways. Once the object and image distances are trustworthy, the mirror relation converts them into the focal length.
- 4
Formula.
1/f = 1/v + 1/u, with u and v the object and image distances from the pole, both measured on the same side for a real image.
- 5
Substitution.
1/f = 1/(20.0 cm) + 1/(30.0 cm).
- 6
Calculation.
1/f = 3/(60.0 cm) + 2/(60.0 cm) = 5/(60.0 cm), so f = 60.0/5 cm = 12.0 cm. The integers 3, 2 and 5 that arise from the common denominator are exact counting numbers and do not limit the significant figures; the precision is set by the two bench readings, each to three significant figures.
- 7
Final answer.
f = 12.0 cm.
- 8
Common trap.
The image distance is the reading at risk. A student who sights the image against the head or the back of the image pin rather than its tip believes parallax has been removed while a constant offset survives, and every trial then carries the same shift. The cure is the matched reference, tip to tip, plus an actual sideways movement of the eye before the reading is accepted.
- 9
Similar NEET-style question.
A concave mirror gives a real image when the object pin is 24.0 cm from the pole and the image pin, set free of parallax, is 12.0 cm from the pole. A student reports the focal length as 18.0 cm. Identify the step that has gone wrong and give the correct focal length. (The student has averaged the two distances; the reciprocals must be added, giving f = 8.00 cm.)
What to remember before solving Experiment 13 Focal Length Parallax questions
Where do students lose marks on Experiment 13 Focal Length Parallax?
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 13 Focal Length Parallax questions from past NEET papers
1 question from NEET 2026. Answers verified against NTA official keys. — click to collapse
NEET 2026
Which of the following measurements require 'index correction' ?
1Measurement of resistance of a wire using meter bridge
2Measurement of gravitational acceleration using simple pendulum
3Measurement of focal length of lenses using optical bench
4Measurement of speed of sound using resonance tube
NTA Answer: Option 3(final)
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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