The equation given below
1/do + 1/di = 1f
is called the Gaussian form of the thin-lens equation. The drawing shows the variables do, di, and f. The drawing also shows the distances x and x', which are, respectively, the distance from the object to the focal point on the left of the lens and the distance from the focal point on the right of the lens to the image. An equivalent form of the thin-lens equation, involving x, x', and f, is called the Newtonian form. Show that the Newtonian form of the thin-lens equation can be written as xx' = f2.

di