Microscopy: Students should be able to carry out calculations involving magnification, real size and image size using the formula:

The idea

The relationship between magnification, image size and real size is given by magnification = image size ÷ real size. Rearranged, image size = magnification × real size and real size = image size ÷ magnification. To use it, measure the image carefully with a ruler, note the magnification of the microscope, and convert all lengths to the same unit before calculating. For example, if a cell image measures 40 mm at ×2000 magnification, the real size is 40 mm ÷ 2000 = 0.02 mm, which is 20 µm. Always give the answer with a sensible unit and check that the real size is smaller than the image size when magnification is greater than 1.

What earns the marks

• Select the correct rearrangement of magnification = image size ÷ real size for the quantity required.
• Substitute measured image size and stated magnification correctly into the chosen equation.
• Convert between units such as millimetres, micrometres and metres so that image size and real size are in the same unit before dividing.
• Calculate accurately and give the answer with an appropriate unit, such as µm for cell dimensions.
• Check the result for sense, for example the real size should be smaller than the image size when magnification is greater than 1.

Common mistakes and how to put them right

• Multiplying when the question asks for real size.

  Put it right: Divide image size by magnification.

• Mixing units, such as dividing a measurement in millimetres by a magnification and quoting the answer in micrometres without converting.

  Put it right: Convert all lengths to the same unit first.

• Forgetting that magnification has no unit, so writing mm or µm after the magnification value.

  Put it right: Treat magnification as a number only.

Exam tips

• Write down the formula and rearrange it before substituting numbers, so the correct operation is clear.
• Convert every length to the same unit, often micrometres for cells, and show the conversion in your working.
• Give the final answer with its unit and check that the real size is smaller than the image size when magnification is greater than 1.