Magnification is the ratio of the image size to the object size. It tells you how many times larger (or smaller) the image is compared to the original object.
The formula
magnification = image height / object height
Magnification has no unit because it is a ratio of two lengths.
If magnification is greater than 1, the image is larger than the object (magnified). If magnification is less than 1, the image is smaller than the object (diminished). If magnification equals 1, the image is the same size as the object.
Using distances
Magnification can also be calculated from distances:
magnification = image distance / object distance
where image distance is the distance from the lens (or mirror) to the image, and object distance is the distance from the lens (or mirror) to the object.
Magnification in ray diagrams
To find the magnification from a ray diagram:
- Measure the height of the image (from the principal axis to the tip of the image arrow).
- Measure the height of the object (from the principal axis to the tip of the object arrow).
- Divide image height by object height.
Sign convention
In IGCSE Physics, magnification is treated as a positive number. The question will tell you separately whether the image is upright or inverted. (Higher-level courses use signed magnification where a negative value indicates an inverted image.)
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