A level Biology exam revision resources written by A level Examiners


The small size of objects viewed through microscopes means that we usually measure them in micrometres. The symbol for micrometres is µm and 1000µm equal one millimetre (mm).

Images produced using microscopes are magnified versions of the originals. AS and A2 exams often involve calculations based around diagrams or photographs of magnified objects. To answer these questions you need to know the relationship between the actual physical size of the object, its observed image size and the level of magnification.   Exam Advice

The best way to remember this relationship is with the mnemonic "old aged man". This can be used to help you remember the triangle shown below.

mnemonic - old aged man

This diagram can help you to write out the equations that can be used in calculations. They are shown below.

Observed size of image = Actual size of object x Magnification

Actual size = Observed size / Magnification

Magnification = Observed size / Actual size

Example 1

If mitochondrion which has a diameter of 2µm and is magnified 30 000 times with an electron microscope the size of the image produced would be calculated as follows.

Observed size of image = Actual size of object x Magnification

Observed size of image = 2 x 30 000

Observed size of image = 60 000µm

This however is not a sensible way to leave the answer, it would be best converted into mm. The final answer would therefore be 60mm

Example 2

If you are asked to calculate the magnification it is essential to make sure both the observed and the actual figures are in the same units!

So if you were given an image of an organelle on an exam paper you measured with a ruler to be 47mm diameter and the question was; "The organelle in the diagram has an actual size of 4µm. Calculate the level of magnification."

Your answer should be

Magnification = Observed size / Actual size

Magnification = 47mm / 4µm

Magnification = 47 000µm / 4µm

Magnification = 11750

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