Question

if the focal length of combination if two thin lens is 80 cm and focal length of one lens is 20 cm then what will be the power of other lens​

Answer 1

As, two thin convex and a concave lens can be combined to form a single lens where combined power is sum of powers of individual lenses.

As focal length of combination=80 cm =>0.8 m

so, combined power=1/f => 1/0.8=> 1.25 D (P)

focal length of 1 lens= 20 cm or 0.2 m , P=1/f =>1/0.2 => 5 D (concave lens) (P1)

Hence, P=P1 + P2

1.25=5 + P2

P2= -3.75 D (Convex Lens)

Hence the power of the other lens is -3.75 D and it is a convex lens.