Question

A student recorded the following data for the values of object distance and corresponding image distance while performing an experiment on real image formation by a convex lens of power + 4 D. Two of these observations are incorrect. Without making any calculations, identify these observations and give reason for your choice.

Answer 1

Readings A and D are incorrect. 

Reason: The focal length of the lens is 25 cm.
(i) In reading A, the object is kept at 30 cm from the lens, i.e. between F and 2F. The image should be formed beyond 2F (50 cm). However, it is shown to be at 20 cm, which is incorrect.
(ii) in reading D, the object is kept beyond 2F, i.e, 50cm. The image should be formed between F and 2F. However, it is also shown to be formed at 60 cm, i.e., beyond 2F. Hence it is incorrect.