Cambridge O Level Biology · Syllabus 5090 · Cells
Magnification
What is Magnification?
The number of times larger an image is than the object it represents, calculated as image size divided by actual size. Because it compares two lengths measured in the same unit, the units cancel and magnification has no unit: it is written as a number of times, such as times four hundred.
This definition is part of the Cells chapter in Cambridge O Level Biology.
Magnification in context
A cell is the smallest structural and functional unit considered in this syllabus. Cell structures are not labels to be memorised in isolation: each one performs work that keeps the cell, and therefore the whole organism, alive. Topic 1 asks you to identify those structures in animal, plant and bacterial cells, to state what each one does, to obtain your own evidence with a light microscope, to record that evidence as a biological drawing, to convert between image size and actual size using magnification, and to explain how specialised cells are built into tissues, organs, organ systems and organisms.
Common mistakes with Magnification
- “\(48 \div 120 = 0.4\), so the magnification is 0.4.” Fix The two lengths were in different units. Convert first: \(48\ \mathrm{mm} = 48\,000\ \mu\mathrm{m}\), then \(48\,000 \div 120 = 400\).
- “The magnification is \(400\ \mu\mathrm{m}\).” Fix A magnification is a ratio of two lengths in the same unit, so it has no unit. Write \(\times 400\).
- “I multiplied by the magnification to get the actual size.” Fix The image is the enlarged one, so the actual size must come out smaller. Divide the image size by the magnification, and check that your answer is smaller than what you started with.
- “The magnification is \(400\ \mu\mathrm{m}\).” Repair Magnification is a ratio of two lengths in the same unit, so the units cancel and it has none. Write \(\times 400\).
Examiner tips on Magnification
- A quick sanity check that costs five seconds. Cells are of the order of micrometres and images on a page are of the order of millimetres, so a magnification should come out in the hundreds or thousands. If your magnification is 0.4, you divided before converting. If an “actual size” comes out larger than the image, you multiplied when you should have divided.
Questions students ask about Magnification
How do you calculate magnification?
Magnification = image size ÷ actual size, and both lengths must be in the same unit before you divide. Convert first: 1 mm = 1000 micrometres, so an image 48 mm wide of a cell 120 micrometres wide gives 48 000 ÷ 120 = ×400. Because magnification is a ratio of two lengths, the units cancel and it has no unit — write ×400, never “400 micrometres”.