A student is using a convex lens of focal length 18 cm to study the image formation by it for the various positions of the object. He observes that when he places the object at 27 cm, the location of the image is at 54 cm on the other side of the lens. Identify from the following diagram the three rays that are obeying the laws of refraction and may be used to draw the corresponding ray diagram.
(A) 1, 2 and 4
(B) 1, 3 and 5
(C) 2, 4 and 5
(D) 2, 3 and 4
Solution
(D) 2, 3 and 4
Ray 2, 3 and 4 are obeying the laws of refraction.
Ray 2 is parallel to the principal axis and passes through the principal focus after refraction.
Ray 3 passes from the optical centre of the lens and emerges without any deviation.
Ray 4 is passing through the principal focus and after refraction from a convex lens emerges parallel to the principal axis.
Ray 1 and 5 cannot pass through the focus after refraction as they are not parallel to the principal axis.
