# Thin lens equation

Lenses form images with 1/f = 1/object distance + 1/image distance.

Physics (university) · Part 09: Induction, light and optics · Card 10 of 11 · Last updated 2026-10-19

## Analogy

Why a magnifying glass can focus a sunbeam.

## Example

**Image distance**

1/di = 1/10 - 1/30
= 2/30
di = 15 cm

## In practice

A lens with f = 10 cm and an object 30 cm away forms an image at 15 cm.

## Practice

### Flashcards

- **What is the thin lens equation?** 1/f = 1/do + 1/di.

## Questions people ask

### What is the sign convention for the lens equation?

In the common textbook convention, real object and image distances are positive, and a positive focal length means a converging lens. A negative image distance means a virtual image on the same side as the object. Check your course's convention before the exam.

### What is magnification?

Magnification m is the image height over the object height, and it equals minus di over do. A negative value means the image is upside down. In the example with f = 10 cm and an object at 30 cm, di is 15 cm and m is minus 0.5, so the image is half-size and inverted.

## Related concepts

- [Electromagnetic wave](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-6)
- [Electromagnetic spectrum](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-7)
- [Snell's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-8)
- [Total internal reflection](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-9)

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