Question

Two thin lenses of power +7D and -5D are placed in contact find the power and focal length of the lens combination

Answer 1

given that -

P1 = 7D

P2 = - 5D

now as they are placed in contact we can find the power of lens combination by the algebraic sum of powers of individual lens..

That is P = P1+P2

P = 7D - 5D

P = 2D

therefore focal length of lens combination will be...

P = 1/f

2 = 1/f

f = 1/2

thus focal length = 0.5 m or 500cm

Hope helped..

Answer 2

P = P1+P2

P = 7D - 5D

P = 2D

therefore focal length of lens combination will be...

P = 1/f

2 = 1/f

f = 1/2

thus focal length = 0.5m