Question

A student placed a candle flame at different distances from a convex lens and focused its image on a screen. He recorded his observation in tabular form as given below:

S. No.	Distance of flame from the lens (cm)	Distance of the image from the lens
1	− 90	+18
2	−60	+20
3	−40	+24
4	−30	+30
5	−24	+40
6	−20	+60
7	−18	+90
8	−12	+120

Analyse the observation table and on the basis of your analysis only, answer the following questions (without doing any calculations):

1. What is the focal length of the convex lens used? Give reason to justify your answer.
2. Which one of the sets of observations is not correct and why?
3. Draw ray diagram to show image formation for any correct set of observation. 

Answer 1

1. When the object (flame) is positioned at -20 cm in the table, the opposite picture is created at +60 cm. This indicates that 20 cm is near the convex lens's focal length since objects positioned outside of 2F create genuine, inverted, and reduced pictures between F and 2F.
2. The observation (S.No. 8: object at -12 cm, image at +120 cm) is incorrect. The convex lens creates a virtual, upright, and enlarged image on the same side as the object when it is positioned at a distance shorter than the focal length (F), such as 12 cm < 20 cm. But according to the provided table, the picture is at +120 cm, which defies the expected behaviour because in this situation, a true image shouldn't appear.
3.