1. The problem statement, all variables and given/known data A gypsy's crystal ball can act as a thick lens whose centre thickness is twice the radius of curvature of the surface. For a 10 cm sphere of glass with n=1.5 what is the focal length? Like this? : http://postimage.org/image/niisv7bq9/ 2. Relevant equations I am not sure if I can simply use the thick lens equation: 1/f = (n-1)*[ (1/r1)-(1/r2)+ ((n-1)*tc)/(n*r1*r2) ] n=1.5 for glass, and tc = diameter of ball =10cm, since 2tc=r1 then r1=5cm. or the Gaussian formula for spherical surfaces: f=(n1*r)/(n2-n1) where n1 is the index of refraction on medium of light origin i.e. n1=1 for air; and n2 is the index on medium entered i.e. 1.5 for glass. 3. The attempt at a solution By the thick lens equation: 1/f= (1.5-1)*[((1.5-1)*10)/(1.5*5*5)]= 0.06666 such that f=15cm. By the Gaussian formula for spherical surfaces: f=(1*5)/(1.5-1)=10 cm Question1: which one is correct and how can you determine this? Question 2: I don't know if r1=-r2 as if one lens were convex (+) and the other concave (-) is taken into account by the formula(s) or if it has to be included at all!!! Please help! Thanks for your time.