Question

A compound lens is made of two lens in contact having powers +12.5 D and -12.5 D. Find the focal length and power of the combination.
B) The magnification produced by a mirror is +1. What does this mean..?

Answer 1

Equivalent power= sum of individual powers. And P=1/f. Power of combination = 0, focal length = infinity. Magnification (linear) is ratio of image size to the object size (considering their signs). Here, magnification is +1 it means size of image is same as size of object, erect image is formed and image is virtual.

Answer 2

power of first lens is +12.5D
power of 2nd lens is -12.5
We know that P=P1+P2
total power = OD
lens will behave as plane glass
focal length of plane glass is always infinite

B)m=+1 signify that image will formed virtual and erect