Physics Lenses
posted by Cole .
A planoconvex lens is to have a focal length of 28.5 cm. If made from fused quartz, what must be the radius of curvature of the convex surface?

Use the socalled "lens maker's formula" and an index of refraction of N = 1.458 for fused quartz.
1/f = (N1)/R1 (for a planoconvex lens with R2 = infinity on one side)
Solve for R1.
http://hyperphysics.phyastr.gsu.edu/hbase/geoopt/lenmak.html