Two thin similar convex glass pieces are joined together front to front with its rear portion silvered such that a sharp image is formed 20 cm from the mirror when the air between the glass pieces is replaced by water ( refractive index 4 / 3 ) then the image formed from the mirror is at a distance of
Can you pls explain this again in words i didn't understood why we took v as 20/2 and why we multiplied 4 while finding focal length