A student focused the image of a candle flame on a white screen by placing the flame at various‘distances from a convex lens. He noted his observations :

(a)From the above table, find the focal length of lens without using lens formula.

(b)Which set of observations is incorrect and why?

©In which case the size of object and image will be same? Give reason for your answer.

(a)From the table of observation, it is clear that for u = 30, v = 30 cm. This means that both u and v must be equal to twice the focal length of the convex lens.

Focal length of the convex lens = 30/2 = 15cm.

(b)The observation (v) is incorrect. For this observation u = 15 = /, i.e., the object is at the focus and the image must be formed at infinity and not 70 cm.

©Since m =v/u , the size of the image will be equal to the size of the object, if v = u i.e., for the observation (iii).