Why is total magnification a product, not a sum?

Each lens magnifies in turn, so the powers multiply: 15 and 10 give 150.

What happens to the field of view at higher power?

The field diameter varies inversely with magnification, so ten times the power means a tenth of the diameter.

S4 Biology Cells, Paper 2: magnification arithmetic and surface area ratio

2026-11-02

Tutor Prep Guide · Primary English

Paper 2 uses a 15x eyepiece and a 1000x objective to practise multiplying powers, real sizes and field diameters.

Focus　Total magnification multiplies the two powers, and the field diameter falls as the magnification rises.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Biology: Cells and Microscopy
Level	S4

02　Questions (10 items)

FoundationA specimen is viewed with a 15× eyepiece and a 10× objective. What is the total magnification?
FoundationAn image measures 25 mm at 1000×. What is the real length in micrometres?
FoundationWhich structure controls what enters and leaves a cell?
FoundationIn which organelle does photosynthesis take place?
IntermediateAt 50× the field diameter is 2.0 mm. What is it at 500×?
IntermediateFor a cube-shaped cell of side 10 micrometres, what is the surface area to volume ratio?
IntermediateA cell looks 8 mm long at 400×. How long will it look at 1000×?
ChallengeCompare the surface area to volume ratios of cube cells of side 30 micrometres and 10 micrometres.
ChallengeThe total magnification is 600× and the eyepiece is 15×. What is the objective power?
Error redoError: a student says a 15× eyepiece with a 10× objective gives 25×. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	150×	Total magnification is eyepiece times objective, so 15 times 10 gives 150.
2	25 micrometres	Dividing 25 mm by 1000 gives 0.025 mm, which is 25 micrometres.
3	The cell membrane	The cell membrane is selectively permeable and decides what may pass.
4	The chloroplast	Chloroplasts hold chlorophyll and use light energy to build sugar from carbon dioxide and water.
5	0.2 mm	Ten times the magnification means one tenth of the field diameter, so 2.0 becomes 0.2.
6	0.6 per micrometre	Surface area is 600 and volume is 1000, so the ratio is 0.6 per micrometre.
7	20 mm	Image length is proportional to magnification, so 8 times 1000 divided by 400 is 20.
8	Three times larger for the smaller cell	The 30 micrometre cell gives 0.2 and the 10 micrometre cell gives 0.6, which is three times larger.
9	40×	Dividing the total magnification by the eyepiece power gives the objective power, so 600 by 15 is 40.
10	150×, because the powers multiply	Adding 15 and 10 gives 25, but total magnification multiplies the two powers.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.