Question

The given ray diagram illustrates the experimental set up for the determination of the focal length of a converging lens using a plane mirror.

1. State the magnification of the image formed.
2. Write two characteristics of the image formed.
3. What is the name given to the distance between the object and optical centre of the lens in the above diagram?

Answer 1

1. Magnification = 1 (image is the same size as the object).
2. Two characteristics: the image is real and inverted (and it coincides with the object/is of the same size). 
3. That distance is the focal length of the lens (OF = f – the object is placed at the principal focus in the diagram). 

With the object at the lens’s principal focus, the lens sends out parallel rays, which the plane mirror reflects back; those rays are then refocused by the lens to form a real image coinciding with the object (hence the same size).