Question

Two thin lenses of power +3.5D and -2.5D are placed in contact find the power and focal length of the lens combination?

Answer 1

total power = 3.5+(-2/5) = 1 D

focal length = 1/total power = 1/1 = 1 m = 100 cm

Answer 2

Answer:

100

Explanation:

Power one of lens = P1 = +3.5D (Given)

Power two of lens = P2 = -2.5D (Given)

Power combination = P1 + P2

Thus,

Power of combination = +3.5 D +(-2.5 D )

= 3.5 D -2.5 D

= 1 D

Power = 1/focal length(m)  

 1 = 1/f

 f = 1/1

 f = 1 metre

Thus, the  f of combination = 1×100 = 100 cm

Therefore, if two thin lenses of power +3.5D and -2.5D are placed in contact then the power and focal length of the lens combination will be 100m.