Question

We are given a concave lens of focal length 15cm.Draw a ray diagram to show the nature,size and position of the image formed when the object is kept at a distance of (a)30cm (b)15cm (c)10 cm

Answer 1

In part a) the image formed at.. Center of curvature

b) the image formed at infinity

Please don't mind these are hand free diagrams

For part c) image formed at the back of the mirror

Answer 2

Focal length f = -15 cm (-ve as it is a concave lens). Image distance v = -10 cm. As we know, 1/f = 1/v – 1/u. Therefore, -1/u = 1/v– 1/f, Therefore 1/u = (1/-10) – (1/-15)= 1/-30. Hence the object distance is -30 cm. Hence the correct answer is a. 30 cm.