Question

a concave lens of focal length 12 cm and a convex lens of focal length 20 CM are kept in contact with each other first find the focal length of the combination second what will be the behaviour of combination

Answer 1

new focal length=( focal length of concave lens) (focal length of convex lens)÷

                                focal length of concave lens+focal length of convex lens

                                =$\frac{(-12)(+20)}{-12+20}$

                                =  -240/8

                                  =  -30

since new focal length is negative behaviour of new combination is that of concave lens