(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).