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

Explanation

(a) When object is placed at the focus of a convex lens of focus 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)/(f)=(1)/(f)`
`(1)/(v)=(1)/(f)+(1)/(u)`
As `f` is negative for a concave lens,
:. `(1)/(v)=(1)/(-f)+(1)/(f) = 0`
`v = oo`
Image is formed at inifinity. The course of rays is shown in Fig.

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