A double convex thin lens made of glass (refractive index `mu = 1.5`) has both radii of curvature of magnitude 20 cm . Incident light rays parallel to the axis of the lens will converge at a distance L such that

Options

A. L=20 cm
B. L=10 cm
C. L=40 cm
D. L=`20/3`cm

Answer

A

Explanation

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