Question

a double convex lens made out of glass has both radius of curvature

Answer 1

Use the lens maker's formula:

1/f=(n-1) [(1/R1)-(1/R2)]

f is focal length.(f=20 cm =0.2 m given).

n is refractive index of material of lens relative to surrounding medium.(n=1.55 given).

R1 is radius of curvature of first surface .

R2 is radius of curvature of second surface

R1=R2=R given.R1 is positive and R2 is negative in Cartesian sign convention .

Then,

(1/f)=(1.55–1)(2/R). Or

(1/0.2)=(0.55)(2/R)=1.1/R. Therefore ,

R=(1.1)(0.2)m=0.22m=22cm.