Two thin equiconvex lenses each of focal length 0.2 m are placed coaxially with their optic centres 0.5m apart. Then find the focal length of the combination.

Explanation

Given, total length of each lens,`f_(1)=f_(2)=0.2m`
Seperation between the lenses,d=0.5 m
`therefore` Focal length of the combination,
`1/F=1/(f_(1))+1/(f_(2))-d/(f_(1)f_(2))=1/0.2+1/0.2-0.5/((0.2)(0.2))=-2.5`
`rArrF=-1/2.5=-0.4m`