Question

A double convex lens made of glass (refractive index `n=1.5`) has the radii of curvature of both the surfaces as `20cm`. Incident light rays parallel to the axis of the lens will converge at a distance `L` such that
A. `L=20cm`
B. `L=10cm`
C. `L=40cm`
D. `L=20/3cm`

Answer 1

(a) Here `n=1.5` as per sign convention followed
`R_(1)=+20cm` and `R_(2)=-20cm`
`:.1/f=(n-1)(1/(R_(1))-1/(R_(2)))=(1.3-1)[1/((+20))-1/((-20))]`
`=0.5xx2/20=1/20impliesf=20cm`
Incident rays travelling parallel to the axis of lens will converge at its second principal focus. Hence `L=+20cm`.