A convex lens of power `2 D` and a concave lens of focal length
`40cm`are kept in contact, find
(a) Power of combination , (b) Equivalent focal length

Answer

B, C

Explanation

(a) Applying
`P=P_1+P_2=P_("convex")+P_("concave")`
`=2+(1)/((-0.4)) , [P("in" D)=(1)/(f(in m))]`
`=2-2.5=-0.5D`
(b) `F=1/P`
`=1/(-0.5)=-2m`
`=-200cm`