Is this formula given in the exam?

No. SPM Physics papers provide no formula sheet, so this must be recalled.

Can magnification be less than 1?

Yes. When m < 1 the image is smaller than the object, diminished. A camera lens photographing a distant building gives a very small m, so the image fits on the sensor.

Does m tell you if the image is upright or inverted?

With the full sign convention a negative m means inverted. In basic SPM work you usually quote the size of m and state upright or inverted from the ray diagram.

Why is m equal to both v/u and hᵢ/hₒ?

The rays from the top of the object form similar triangles about the lens centre. The ratio of heights equals the ratio of distances, so the two expressions are the same.

Linear magnification Formula, SPM Physics

Q: Can magnification be less than 1?

Yes. When m < 1 the image is smaller than the object, diminished. A camera lens photographing a distant building gives a very small m, so the image fits on the sensor.

Q: Does m tell you if the image is upright or inverted?

With the full sign convention a negative m means inverted. In basic SPM work you usually quote the size of m and state upright or inverted from the ray diagram.

Q: Why is m equal to both v/u and hᵢ/hₒ?

The rays from the top of the object form similar triangles about the lens centre. The ratio of heights equals the ratio of distances, so the two expressions are the same.

Linear magnification

m = v / u = hᵢ / hₒ

Not given in the exam, recall it.

What it is for

Linear magnification is used in Light and Optics (SPM Physics Form 4). Keep the SI units in every line of working. Light and Optics

Symbols and SI units

Symbol	Meaning	SI unit
`m`	linear magnification (no unit, ratio)	,
`v`	image distance	m
`u`	object distance	m
`hᵢ`	image height	m
`hₒ`	object height	m

Rearrangements

v = mu
hᵢ = m × hₒ
u = v / m

Worked example

An image forms at v = 30 cm for an object at u = 15 cm. m = v / u = 30 / 15 = 2.0 (the image is twice as tall).

Common trap

m is a ratio with no unit. m > 1 means enlarged, m < 1 means diminished. Keep u and v in the same unit.

Understanding linear magnification

Linear magnification m = v / u = hᵢ / hₒ compares image to object. m is a pure ratio with no unit. v is image distance and u is object distance (both in metres, SI); hᵢ is image height and hₒ object height (also in metres).

The two ratios are equal because similar triangles link the distances to the heights: an image formed twice as far away is twice as tall.

Rearrange as needed: hᵢ = m × hₒ gives the image height, v = mu gives the image distance, and u = v / m gives the object distance. Keep u and v in the same unit; because m has no unit, the units cancel and only the ratio matters.

A worked example, step by step

An object hₒ = 2 cm tall sits at u = 10 cm from a lens; its image forms at v = 40 cm. Find the magnification and the image height.

Magnification:

m = v / u = 40 / 10 = 4.0

Image height:

hᵢ = m × hₒ = 4.0 × 2 = 8 cm

So the magnification is m = 4.0 (no unit) and the image is hᵢ = 8 cm tall (2 significant figures). Because m is greater than 1, the image is enlarged, four times the object's height, which fits the object being much closer to the lens than the image.

Common mistakes and how this is tested

The core trap: m is a ratio with no unit; m > 1 means enlarged, m < 1 means diminished, so keep u and v in the same unit.

Other common errors:

Writing a unit on m, such as "m = 4 cm".
Inverting the ratio, using u/v instead of v/u.
Multiplying by the wrong height when finding hᵢ or hₒ.

Questions ask you to calculate m, hᵢ or v, to define linear magnification, or to explain whether an image is enlarged or diminished from a value of m.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)

Written by the spmphysics.com.my editorial team.· Updated 5 Sept 2026