Question

The diagram shows an object OA and its image IB formed by a lens. The image is same size as the object. 

a) Complete the ray diagram and locate the focus of lens by labelling it as F. 

b) State whether the lens is convex or concave. 

Show it in the diagram.

Answer 1

a) 

1. Optical centre goes undeviated, therefore to find the optical centre, join A to B to meet the line 01 at the point P which gives the position of optical centre of the lens. 

2. Draw a line CP through P perpendicular to the line 01 to represent lens. 

3. Draw another ray AC from the point A parallel to the principal

axis 01 to meet the lens line CP at a point C. 

4. This ray AC will reach the image point B while passing through the focus, therefore join C to B to meet line 01 at a point F which is the focus of the lens. The completed ray diagram is shown above. 

b) Since the image is real and inverted, the lens is convex. 

c) Since the size of object and image of equal, the object must be at a distance of twice the focal length, i.e., at 2F₂.