Chapter 30, Problem 18A

Depending on the location of Percy relative to the focal point of the converging lens (only one of the symmetrical focal points is shown here), the image may be real or virtual. The size of the image varies for object positions A, B, and C. Rank the image sizes from largest to smallest. (You may want to draw ray diagrams on a separate piece of paper.)

To determine

To rank: The image distance if object is moved away from the convex lens.

Explanation of Solution

Introduction:

A convex lens is also called converging lens as it converges all the beam of light incident on it to a single point called focus.

As a convex lens converges the beam of light incident on it, if the object is placed farther from the lens, the image moves closer to it. For example let’s consider two cases 1) when the object is at infinity, the image formed by the lens is at its focus 2) when the object is at focus, the image is formed at infinity.

These two examples clearly make it easy to understand that as the object is shifted towards the convex lens, it image shifts away from the lens and is also formed magnified. In the same way, as the object is shifted away from the lens, its image comes closer and closer to the lens and is smaller in size.

If the object is placed beyond the focus of the converging lens, for all the positions of the object, the image formed is real and inverted. It is always virtual and magnified if the object is placed between optical centre and focus of the lens.

Conclusion:

Hence, based on the above facts, the image formed would be largest when the object is placed at C, then at B and then smallest at A.