Formula
Magnification by mirror
m = -v/u = h_image/h_object. Negative m: inverted image. |m| > 1: magnified; |m| < 1: diminished.
-- NCERT Class 12 Physics, Ch. 9, p. 226Magnification
8 MCQs1 revision card9-step worked example
Source: NCERT OpticsOfficial key: NTA-verifiedLast updated: 25 Sep 2026
Magnification, explained for NEET
The sign of m is not decoration. For a spherical mirror, m = −v/u, and the minus sign in front is part of the formula — students who compute v/u and then attach a sign "by reasoning about whether the image looks upright" get it right about half the time and lose four marks the other half.
Magnification is defined as the ratio of image height to object height: m = h′/h. For mirrors, NCERT Class 12 Physics Chapter 9 (page 223) derives from similar triangles that this ratio also equals −v/u, with u and v signed under the Cartesian convention — origin at the pole, distances measured positive in the direction of incident light.
Read the result, don't reason it out afresh. A negative m means the image is inverted; a positive m means erect. |m| > 1 means enlarged, |m| < 1 diminished. That is the whole interpretation table, and it follows from the signed arithmetic without any separate ray-diagram argument.
Where this turns costly: a real object in front of a concave mirror sits at negative u. If the image is also real it forms on the same side, so v is also negative. Then −v/u has a negative divided by a negative inside, and the leading minus makes m negative — inverted, as the ray diagram says. Drop the leading minus and you report an erect real image, which is impossible for a single concave mirror and which every paper offers as an option.
In NEET this appears two ways: as a direct calculation (given two of u, v, f, find m), and as a one-line inference (given m and one distance, find the other). Both are single-formula questions once the convention is fixed. The failure is never the algebra.
Watch-out: fix your sign convention before touching the numbers, and carry it through u, v, f and m without exception. Switching halfway is the error that produces a plausible wrong option.
Can you answer these Magnification 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
For a spherical mirror, the linear magnification produced is given by
Show answer and why every option is right or wrong
Answer: D. D is correct. Magnification is image height over object height, and for a spherical mirror this equals −v/u, as derived from similar triangles in NCERT Class 12 Physics Chapter 9, page 223.
Why A is wrong: A is wrong because it inverts the distance ratio. The image distance appears in the numerator, not the denominator.
Why B is wrong: B is wrong because it drops the leading minus sign. Without it a real, inverted image formed by a concave mirror would be reported as erect.
Why C is wrong: C is wrong because it inverts the height ratio as well as the distance ratio; magnification is image height over object height, not the reverse.
MCQ 2Easy RecallPractice
A mirror produces an image for which the magnification is −0.50. The image is
Show answer and why every option is right or wrong
Answer: B. B is correct. The negative sign of m means inverted, and |m| = 0.50 < 1 means diminished. NCERT Class 12 Physics Chapter 9, page 223.
Why A is wrong: A is wrong on both counts: a negative m is inverted, not erect, and |m| < 1 means diminished, not enlarged.
Why C is wrong: C is wrong because it reads the magnitude correctly but ignores the sign. Negative m means inverted.
Why D is wrong: D is wrong because it reads the sign correctly but misreads the magnitude. |m| = 0.50 is less than 1, so the image is smaller than the object.
MCQ 3Easy RecallPractice
In the relation m = −v/u applied to a spherical mirror, the distances u and v are measured from
Show answer and why every option is right or wrong
Answer: A. A is correct. Under the Cartesian sign convention used in NCERT Class 12 Physics Chapter 9 (page 223), all distances for a spherical mirror are measured from the pole, taken as the origin, and are positive in the direction of incident light.
Why B is wrong: B is wrong because the focus is itself located by a distance measured from the pole; it is not the origin.
Why C is wrong: C is wrong because the object position is what u measures; it cannot also be the origin.
Why D is wrong: D is wrong because the centre of curvature is a reference point for ray construction, not the origin of the distance convention.
MCQ 4Direct ApplicationPractice
An object is placed 3.00 × 10⁻¹ m in front of a mirror, and a real image forms 6.00 × 10⁻¹ m in front of it. Taking the incident light as travelling in the negative direction relative to the pole, the magnification is
Show answer and why every option is right or wrong
Answer: B. B is correct. With the object at u = −3.00 × 10⁻¹ m and the real image on the same side at v = −6.00 × 10⁻¹ m, m = −v/u = −(−0.600)/(−0.300) = −2.00. Negative m means inverted, consistent with a real image. NCERT Class 12 Physics Chapter 9, page 223.
Why A is wrong: A is wrong because it drops the leading minus in m = −v/u, reporting an erect real image — which a single mirror cannot produce.
Why C is wrong: C is wrong because it inverts the ratio to −u/v. The image distance belongs in the numerator.
Why D is wrong: D is wrong because it both inverts the ratio and drops the leading sign, compounding two separate errors.
MCQ 5Direct ApplicationPractice
A mirror forms an image of an object for which the magnification is −3.00. The object is 2.0 × 10⁻² m tall. The image height is
Show answer and why every option is right or wrong
Answer: D. D is correct. Since m = h′/h, the image height has magnitude |m| × h = 3.00 × 2.0 × 10⁻² m = 6.0 × 10⁻² m, and the negative m makes it inverted. NCERT Class 12 Physics Chapter 9, page 223.
Why A is wrong: A is wrong on both counts: dividing by 3 instead of multiplying (2.0 × 10⁻²/3.00 ≈ 6.7 × 10⁻³ m), and ignoring the negative sign that marks the image as inverted.
Why B is wrong: B is wrong because it divides the object height by the magnification instead of multiplying, giving a value a ninth of the correct size — though it does read the sign correctly.
Why C is wrong: C is wrong because it gets the magnitude right but reads a negative magnification as erect. Negative means inverted.
MCQ 6Direct ApplicationPractice
A concave mirror forms an image at a distance whose magnitude is one-quarter of the object distance. The magnification is
Show answer and why every option is right or wrong
Answer: A. A is correct. For a concave mirror with a real object and a real image, u and v carry the same sign, so −v/u is negative; with |v| = |u|/4 the magnitude is 0.250, giving m = −0.250. NCERT Class 12 Physics Chapter 9, page 223.
Why B is wrong: B is wrong because it inverts the ratio. The image distance is one-quarter of the object distance, so the ratio v/u has magnitude 0.250, not 4.00.
Why C is wrong: C is wrong because the magnitude is right but the leading minus in m = −v/u has been dropped — the error that turns a real inverted image into an impossible erect one.
Why D is wrong: D is wrong because it inverts the ratio (using u/v instead of v/u) and drops the leading sign.
MCQ 7CalculationPractice
An object is placed at a distance of magnitude 1.5 × 10⁻¹ m from a concave mirror of focal length of magnitude 1.0 × 10⁻¹ m. Using the Cartesian convention with the object at u = −1.5 × 10⁻¹ m and f = −1.0 × 10⁻¹ m, the magnification is
Show answer and why every option is right or wrong
Answer: C. C is correct. From 1/v + 1/u = 1/f: 1/v = 1/(−0.10) − 1/(−0.15) = −10 + 6.67 = −3.33 m⁻¹, so v = −0.30 m. Then m = −v/u = −(−0.30)/(−0.15) = −2.0. NCERT Class 12 Physics Chapter 9, page 223.
Why A is wrong: A is wrong because it reports the ratio of the given distances (f to u, or similar) rather than solving the mirror formula for v first. The image distance must be found before magnification.
Why B is wrong: B is wrong because it both skips solving for v and drops the leading sign.
Why D is wrong: D is wrong because +0.67 is f/u = 0.10/0.15, a ratio of the wrong quantities. The mirror formula gives v = −0.30 m, and m = −v/u = −(−0.30)/(−0.15) = −2.0.
MCQ 8Concept TrapPractice
A student computes v/u for a concave mirror with a real object and a real image, finds the result positive, and reports the magnification as positive. The reported answer is
Show answer and why every option is right or wrong
Answer: C. C is correct. The quotient v/u is indeed positive here, but magnification is −v/u, not v/u. Omitting the leading minus converts a genuinely inverted image into a reported erect one. NCERT Class 12 Physics Chapter 9, page 223.
Why A is wrong: A is wrong because it stops one step early. The quotient being positive is correct arithmetic; the error is in the formula, which has a minus in front of that quotient.
Why B is wrong: B is wrong because a single concave mirror never forms an erect real image — that combination is physically impossible, which is precisely what makes the dropped sign detectable.
Why D is wrong: D is wrong because inverting the ratio is a second, separate error rather than the fix. The distance ratio v/u is correct as written; only the leading sign is missing.
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Magnification: quick recall before you leave
How do you solve a Magnification question? A worked example
- 1
Given.
A concave mirror of focal length of magnitude 2.0 × 10⁻¹ m. An object is placed so that its distance from the pole has magnitude 3.0 × 10⁻¹ m. Object height 4.0 × 10⁻² m. Using the Cartesian convention with light incident from the left:
u = −3.0 × 10⁻¹ m,f = −2.0 × 10⁻¹ m. - 2
Required.
The magnification
m, and the image height with its orientation. - 3
Concept.
Magnification for a spherical mirror is the signed ratio
m = −v/u, which also equalsh′/h. The image distance is not given, so it must first be obtained from the mirror formula. One sign convention is fixed at the start and carried through every quantity. - 4
Formula.
1/v + 1/u = 1/fto findv; thenm = −v/u; thenh′ = m × h. - 5
Substitution.
1/v = 1/f − 1/u = 1/(−2.0 × 10⁻¹) − 1/(−3.0 × 10⁻¹) - 6
Calculation.
1/v = −5.0 + 3.33 = −1.67 m⁻¹v = −6.0 × 10⁻¹ mm = −v/u = −(−6.0 × 10⁻¹)/(−3.0 × 10⁻¹) = −2.0h′ = m × h = (−2.0)(4.0 × 10⁻² m) = −8.0 × 10⁻² m
The2appearing in the ratio is a computed result, not a counting constant; the only exact quantities here are the signs themselves, which are convention rather than measurement and do not enter the significant-figure count. Both given lengths carry two significant figures, so the answers are reported to two. - 7
Final answer.
m = −2.0. The image height is8.0 × 10⁻² m, and the negative magnification means the image is inverted. It is also real, forming on the same side as the object, and enlarged since|m| > 1. - 8
Common trap.
Computing
v/u = (−6.0 × 10⁻¹)/(−3.0 × 10⁻¹) = +2.0and reportingm = +2.0. The arithmetic is right and the formula is wrong:m = −v/u, so the leading minus flips it to−2.0. A reported+2.0claims an erect real image from a concave mirror, which cannot happen — and+2.0will be sitting in the options. - 9
Similar NEET-style question.
An object 5.0 × 10⁻² m tall stands 4.0 × 10⁻¹ m from a concave mirror of focal length of magnitude 1.0 × 10⁻¹ m. Find the magnification and state whether the image is erect or inverted, enlarged or diminished.
What to remember before solving Magnification questions
Which Magnification formulas do you need for NEET?
1 formula — click to collapse
Mirror formula
Spherical mirror formula. Sign convention: distances from pole, positive in direction of incident light.
| Symbol | Quantity | SI Unit |
|---|---|---|
| v | image distance | m |
| u | object distance | m |
| f | focal length | m |
| m | magnification | - |
Valid when
- Spherical mirror, paraxial rays
- Sign convention used consistently
More in Optics: 9 exam traps and mistakes · 10 formulas · 7 question patterns from its other lessons.
Magnification 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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