A light source is located to the left of two lenses. You can click-drag the left lens through a limited range of x values. The right lens will move in tandem. Find the focal length of these lenses.
PS-- A text field is used to display the position of the left lens for illustration purposes. The text field calculation can use the following variables: x, y, f, n. However, some objects do not set all these values. For example, a mirror would not set the index of refraction, n, and an aperture would not set the focal length, f.