Physica
posted by fa 1 .
A contact lens is made of plastic with an index of repraction of 1.5 , The lens has an outer radius of eurvature ( convex) of 2 cm and an inner radius of curvature (concave ) of 4 cm .the focal length of the lens is;

Use the Lens Maker's formula, which is
1/f = (n1)[(1/R1)  (1/R2)]
= 0.5 [(1/2) (1/4)]
= 0.5 [1/4) = 1/8 cm^1
f = 8 cm
where f is the focal length, n is the index of refraction, and R1 and R2 are the radii of curvature of the two sides of the lens. (R1 is convex and R2 is concave)
ref.:
http://scienceworld.wolfram.com/physics/LensMakersFormula.html