Question

An object is held at the principal focus of a concave lens of focal length f. Where is the image formed?

Answer 1

1/f = 1/v - 1/u = 1/v = 1/f + 1/u

Here, u = - f and for a concave lens f = -f

Therefore, 1/v = - 1/f - 1/f => v = - f/2

That is image will be formed between optical centre and focus of lens; towards the side of the object.