CBSE Class 12 · 2008 · Geometrical OpticsVery Short Answer

An object is held at the principal focus of a concave lens of focal length

F

. Where is the image formed?

Answer: At a distance $F /2$ from the lens, between the optical centre and the focus (virtual, erect, diminished).

Solution

With

u = - F

and focal length

- F

for the concave lens,

\frac{1}{v} = \frac{1}{- F} + \frac{1}{- F} = - \frac{2}{F} \Rightarrow v = - \frac{F}{2}

The image lies at

F /2

on the same side as the object.

CBSE Class 12 Board · 2008 · Subjective