A spherical air lens of radii `R_1=R_2=10cm`
Is cut in a glass `(mu=1.5)` cylinder. Determine the focal length and nature of air lens. If a liquid of refractive index 2 is filled in the lens, what will happen to its focal length and nature?

Explanation

`(1)/(f_(air l ens)=(``_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))((2)/(10))`
`f_(air l ens)=-15cm`
`(1)/(f_l)=(``_amu_l-1)((1)/(R_1)-(1)/(R_2))`
`=((``_amu_g_l)/(``_amu_g)-1)((1)/(R_1)-(1)/(R_2))`
`=((2)/((3)/(2))-1)((1)/(10)-(1)/(-10))`
`=((1)/(3))((2)/(10))`
`f_1=15cm`
Nature of lens changes.