Magnification Formula (Cambridge O Level Biology)

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Cara Head

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Cara Head

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Magnification Formula

Calculating magnification and specimen size using millimetres as units

  • Magnification is calculated using the following equation:

Magnification = Image size ÷ Actual size

  • A better way to remember the equation is using an equation triangle:

Magnification EquationMagnification equation

  • Rearranging the equation to find things other than the magnification becomes easy when you remember the triangle - whatever you are trying to find, place your finger over it and whatever is left is what you do, so:
    • Magnification = image size / actual size
    • Actual size = image size / magnification
    • Image size = magnification x actual size

Remember magnification does not have any units and is just written as ‘x 10’ or ‘x 5000’

Worked example

An image of an animal cell is 30 mm in size and it has been magnified by a factor of x 3000. What is the actual size of the cell?

To find the actual size of the cell:

Worked Example Using Magnification Equation

Worked example using the magnification equation

Examiner Tip

To ensure you do not lose marks when answering exam questions on this skill:

  1. Always look at the units that have been given in the question - if you are asked to measure something, most often you will be expected to measure it in millimetres NOT in centimetres - double check the question to see! Remember to bring a ruler to the exam!
  2. Learn the equation triangle for magnification and write it on the page straight away
  3. Don’t forget that magnification has NO UNITS - students often lose a mark because they put one in

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding