The radii of curvatures of a double convex lens are 15 cm and 30 cm, and its refractive index is 1.5. Then its focal length is -

Options

A. `22` cm
B. `25` cm
C. `30` cm
D. `32` cm

Answer

A

Explanation

According to lens maker.s formula,
`(1)/(f)=(mu-1)((1)/(R_(1))-(1)/(R_(2)))`
For double convex lens, `R_(1)=+R,R_(2)=-R`
`(1)/(20)=(1.55-1)[(1)/((+R))-(1)/((_R))]implies(1)/(20)=(0.05)((2)/(R))`
R= `22` cm