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How to use this calculator
Choose the formula (if there is more than one), choose what you want to find, then fill in the other values using the units shown. The calculator rearranges the formula, puts your numbers in, and shows every step.
How it's calculated
Lens and mirror formula: 1/f = 1/u + 1/v. Magnification m = v / u. Real objects and real images are taken as positive distances.
Worked example
An object 30 cm from a lens gives a real image 60 cm away. 1/f = 1/30 + 1/60 = 3/60, so f = 20 cm.
Frequently asked questions
What is a virtual image?
One that cannot be shown on a screen. With this sign convention it has a negative image distance.
Does this work for mirrors too?
Yes, the same formula applies.
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Last updated October 2026. Results are for learning and guidance; always check important work. Calculations run in your browser and are not stored.