Question

Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.

Answer 1

The lens formula is 1/u + 1/v = 1/f, where u is the subject distance, v is the object distance, and f is the focal length.

You’re taking the case where u = f.

Under these circumstances, v is infinity.

This is exactly the converse of the normal situation where you take a photo of something a long way away, and the image is formed effectively at the focal point.