Question

Draw ray diagrams to represent the nature, position and relative size of the image formed by a convex lens for the object placed:

1. at 2F₁,
2. between F₁ and the optical centre O of the lens.

Which of the above two cases shows the use of a convex lens as a magnifying glass? Give reasons for your choice.

Answer 1

a. When an object is placed at 2F₁ , the image so formed by the convex lens is at 2F. This image is real, inverted and of the same size as the object, as shown in the figure. 

 

b. When the object is placed between F₁ and the optical centre O of the lens, the image formed by the convex lens is behind the object (left of the lens). This image is virtual, magnified and erect as shown in the figure. 

 

It is used as a magnifying glass. The reason being, the image of an object that is placed between F₁ and the optical centre is virtual and magnified, which is the requirement of a magnifying glass.