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;

Answer 1

Use the Lens Maker's formula, which is

1/f = (n-1)[(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

Answer 2

20cm