How to calculate magnification
A lens can form an image that is bigger or smaller than the object. Magnification tells you how many times bigger:
magnification = image height ÷ object height
- Magnification has no unit, because it is a ratio of two heights
- Image height and object height must be in the same unit
An object 6 mm tall that forms an image 15 mm tall has a magnification of 15 ÷ 6 = 2.5. A magnification less than 1 means the image is smaller than the object.
Because it is a ratio, you don't need to convert to metres as long as both heights use the same unit. The solver converts to metres anyway, so the working stays consistent, and shows the answer in cm and mm too.
Common mistakes that cost marks
- Giving magnification a unit. It is a ratio, so it has no unit.
- Dividing the wrong way. Image height goes on top.
- Using different units for the two heights. Convert so both are in mm or both in cm.
Practice questions
Try each one on paper first, then reveal the worked answer.
Q1An object 6 mm tall forms an image 15 mm tall. Calculate the magnification.[2 marks]
- magnification = image height ÷ object height
- = 15 ÷ 6
- magnification = 2.5
Q2A magnifying glass has a magnification of 4. The object is 1.2 cm tall. Calculate the image height.[2 marks]
- image height = magnification × object height
- = 4 × 1.2
- image height = 4.8 cm
Q3A lens forms an image 9 mm tall with a magnification of 0.5. Calculate the object height.[2 marks]
- object height = image height ÷ magnification
- = 9 ÷ 0.5
- object height = 18 mm
Endless practice
A new question every time, marked instantly. Work it out on paper, then type your answer.
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Frequently asked questions
What is the equation for magnification?
Magnification = image height ÷ object height. It has no unit.
What does a magnification less than 1 mean?
The image is smaller than the object (diminished).
Is magnification in Combined Science?
In AQA, lenses and magnification are in GCSE Physics (separate science) only.