Two thin similar convex glass are joined together front to front, with its rear portion silvered such that a sharp image is formed `0.2m` for an object at infinity. When the air between the glass pieces is replaced by water `(mu=4/3),` find the position of image.

Answer

- 12 cm

Explanation

It is just like a concave mirror.
`|f|=0.2 m`
`:. |R|=0.4 m`
Focal length of this equivalent mirror is
`1/F=(2(mu_2//mu_1))/R_2-(2(mu_2//mu_1-1))/R_1 ("extra points")`
`=(2(4//3))/-0.4-(2(4//3-1))/(+0.4)`
or `F=-0.12 m or -12 cm`