Question

a converging lens of focal length 25 andndivergijg lens of 10 cm are placed in close contact with one another . find focal length amd power of the combination

Answer 1

Focal length of converging lens (convex lens) = 25cm
Focal length of diverging lens (concave lens) = 10cm. = -10cm 

Focal length of combination = 1/f1 + 1/f2 + ......... + 1/fn
                                             = 1/25 - 1/10
                                             = 2/50 - 5/50
                                             = -3/50
1/f = -3/50
f = -50/3cm = -16.66cm

P = 1/f(in m)
P = 1/-0.16
P = -6.2 D