Problem 12

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.

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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.