In figure, an air lens of radius of curvature of each surface equal to `10 cm` is cut into a cylinder of glass of refractive index `1.5.` The focal length and the nature of lens are

Options

A. 15 cm, concave
B. 15 cm, convex
C. `infty`, neither concave nor convex
D. 0, concave

Answer

A

Explanation

`(1)/(f)=(._amu_a-1)((1)/(R_1)-(1)/(R_2))=((1)/(``_amu_g)-1)((1)/(R_1)-(1)/(R_2))`
`=((1)/((3)/(2))-1)((1)/(10)-(1)/(-10))=(-(1)/(3))((1)/(5))`
`f=-15cm`
15 cm concave.