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 a ray diagram to show image formation for any correct set of observations.

Answer 1

1. As observation ‘4’ indicates that the item is at the center of curvature and the image is at the same distance from the pole, or center of curvature, the convex lens’s focal length is 15 cm. Therefore, f = `R/2`.
2. When an object is between the focus and the pole image, a virtual image is formed, but observation 8 is incorrect.
3.