The thin lens equation –
– where
is the focal length
is the distance of the object from the lens
is the distance of the image from the lens
can be easily derived using properties of similar triangles.
The triangles CAB and CDE are similar, so
(1)where O is the size of the object and I is the size of the image.
The triangles GBF and GFC are also similar so
Hence
From (1)
hence