Question

A convex lens of power 5.0 diopter and a concave lens of power 7.5 diopter are placed in a close contact with each other what is the power and focal length of a combination explain whether the combination act like a converging or diverging lens

Answer 1

Answer:

A convex lens has a positive power whereas a concave lens always ha a negative power

therefore

power of convex lens = +5.0 D

power of concave lens = -7.5 D

Whenever two lens are placed in contact with each other their power adds up( appropriate sign conventions should be taken )

therefore the effective power of the combination of lens  = +5.0 - 7.5

                                                                                                = - 2.5 D

power = 1/focal length in metre    

-2.5 D = 1/ focal length

focal length = -1/2.5

                    = -40 cm

the combination acts as a diverging lens because since the power is negative the lens is a concave lens and all concave lens are diverging lens.