Magnification questions need three things: an accurate measurement, the right formula and matching units. Measure images with an on-screen ruler, work out magnification and actual size, use scale bars, and practise converting millimetres and micrometres. Fresh numbers every time.
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The image measured accurately in millimetres between the marked points.
Magnification = image size ÷ actual size; actual size = image size ÷ magnification.
Both sizes in the same units before dividing: 1 mm = 1000 µm.
Magnification written with × and no unit; sizes given with their unit.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.