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**1. Homework Statement**

Derive the optical length of the incident light being reflected back to the surface of the lens with radius r1.

**2. Homework Equations**

i have assumed that the surfaces can be given as r^2 = y^2+z^2

there are 2 surfaces of the lens with radii r1 and r2

**3. The Attempt at a Solution**

is it simply subtracting the expression for r1 from the expression for r2?

i have attemped using Fermats principle but cannot get very far