A point object moves towards a stationary thin convex lens along the principal axis with a constant speed of 2cms−1. At a certain instant, the object is 24cm in front of the lens whose focal length is 8cm.
(a) Derive a general expression relating the speed vi of the image to the speed vo of the object for a thin lens, assuming motion strictly along the principal axis.
(b) Using your expression, calculate the speed with which the image is moving (magnitude and direction) at the instant described.
(c) State whether the image is moving towards or away from the lens and give a brief reason based on your calculation.