Thin lens equation
Lenses form images with 1/f = 1/object distance + 1/image distance.
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Analogy
Why a magnifying glass can focus a sunbeam.
Example
1/di = 1/10 - 1/30= 2/30di = 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.