Why is the field diameter inversely proportional to magnification?

Higher power shows a smaller patch of the specimen, so the diameter shrinks in proportion.

Why does the surface area to volume ratio matter?

A larger ratio means faster exchange, so small cells move materials more efficiently.

S4 Biology Cells, Paper 3: magnification, field of view and organelles

2026-11-02

Paper 3 uses a 20x eyepiece with a 40x objective to practise multiplying powers, field diameters and organelle functions.

Focus　Total magnification multiplies the two powers, field diameter falls as power rises, and each organelle has its own job.

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 20× eyepiece and a 40× objective. What is the total magnification?
FoundationAn image measures 12 mm at 500×. What is the real length in micrometres?
FoundationWhich organelle is the main site of respiration in a cell?
FoundationWhich structure is found in plant cells but not in animal cells?
IntermediateAt 100× the field diameter is 1.8 mm. What is it at 400×?
IntermediateFor a cube-shaped cell of side 2 micrometres, what is the surface area to volume ratio?
IntermediateA specimen looks 4 mm long at 200×. How long will it look at 800×?
ChallengeThe total magnification is 1000× and the eyepiece is 10×. What is the objective power?
ChallengeA specimen is 0.05 mm long. How long is its image at 200×?
Error redoError: a student says the field of view grows when a higher power objective is used. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	800×	Total magnification is eyepiece times objective, so 20 times 40 gives 800.
2	24 micrometres	Dividing 12 mm by 500 gives 0.024 mm, which is 24 micrometres.
3	The mitochondrion	Mitochondria carry out aerobic respiration and release usable energy from food.
4	The cell wall, along with chloroplasts and a large vacuole	Plant cells have a cellulose cell wall, while animal cells only have a membrane.
5	0.45 mm	Four times the magnification leaves a quarter of the field diameter, so 1.8 becomes 0.45.
6	3 per micrometre	Surface area is 24 and volume is 8, so the ratio is 3 per micrometre.
7	16 mm	Image length is proportional to magnification, so 4 times 800 divided by 200 is 16.
8	100×	Dividing the total magnification by the eyepiece power gives the objective power, so 1000 by 10 is 100.
9	10 mm	Real length times magnification gives image length, so 0.05 times 200 is 10.
10	The field shrinks because diameter is inversely proportional to magnification	Higher power shows a smaller area, so the field diameter shrinks rather than grows.

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.