Calculate the position of the image of an object when placed at
(a) focus of a convex lens.
(b) focus of a concave lens.

Explanation

(a) When object is placed at the focus of a convex lens of focal length `f`, `u = - f, v = ?`
From lens formula `(1)/(v) - (1)/(u) = (1)/(f)`
`(1)/(v) = (1)/(f) + (1)/(u) = (1)/(f) - (1)/(f) = 0, :. v = oo`
`:.` Image is formed at infinity. The course of rays is shown in Fig.

(b) When object is placed at the focus of a concave lens of focal length `f, u = + f, v = ?`
From lens formula, `(1)/(v) - (1)/(u) = (1)/(f)`
`(1)/(v) = (1)/(f) + (1)/(u)`
As `f` negative fro a concave lens.
`:. (1)/(v) = (1)/(-f) + (1)/(f) = 0`
`v = oo`
Image os formed at infinity. The course of ray is shwon in Fig.