International A Level Biology / Cell structure

Magnification calculations and units

Edexcel International A Level Biology (YBI11), chapter 7: Cell structure, specification point 3.7, 3.8. Free lesson.

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Magnification compares an image size with the real size of an object. Correct unit conversions and a quick sense-check help you avoid unrealistic answers.

What the magnification equation means

The equation is magnification = image size ÷ actual size. Magnification tells you how many times bigger the image is than the real object. Before dividing, make sure image size and actual size are in the same unit. M is magnification. It has no unit and is written with a × sign. I is image size, measured on the photo or drawing, usually in mm. A is actual size, usually in µm for cells or nm for organelles.

How to rearrange the equation

Write the equation with the unknown you need on its own before putting in the numbers. This helps you choose the right calculation and avoid mixing up image size and actual size.

How to convert between mm, µm and nm

Each step to a smaller unit makes the number 1000 times bigger. Going to a larger unit makes the number 1000 times smaller. Convert to the unit the question asks for. 1 mm = 1000 µm. 1 µm = 1000 nm. So, multiply by 1000 for each step down from mm to µm to nm. Divide by 1000 for each step up.

How to use a scale bar

A scale bar shows how much real distance a measured length in the image represents. Use the bar's measured length, not the object's length, to find magnification. Convert the bar measurement into the unit used for its real length first. Magnification = measured length of scale bar ÷ actual length represented by the bar. Then use A = I/M to find the object's actual size, with image size and actual size in the same unit.

How to check your answer

A calculated result should be sensible for the object and the microscope. For example, a cell much smaller than the resolving limit of a light microscope is a warning that the units may be wrong. Check that the final answer has the unit asked for. Check that the magnification has a × sign but no unit. Round to the requested number of significant figures. Two significant figures is not the same as two decimal places.

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