The radius of curvature of curved surface at a thin planoconvex lens is 10 cm and the refractive index is 1.5. If the plane surface is silvered, then the focal length will be,

Options

A. 5 cm
B. 10 cm
C. 15 cm
D. 20 cm

Answer

A, C

Explanation

According to len.s maker formula, `(1)/(f)=(n-1)((1)/(R_(1))-(1)/(R_(2)))`
`R_(1)=oo:R_(2)=-R`
`thereforef=(R)/((n-1))(R=10cm,n=1.5)`
`f=(10)/((1.5-1))=20cm`