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CBSE Class 12 · 2003 · Geometrical OpticsVery Short Answer

An object is placed at the focus of a concave lens. Where will its image be formed?

Solution

For a concave lens

f = - f

and the object is at its focus,

u = - f

.

\frac{1}{v} - \frac{1}{u} = \frac{1}{f} \Rightarrow \frac{1}{v} = \frac{1}{- f} + \frac{1}{- f} = - \frac{2}{f}

v = - \frac{f}{2}

The image is virtual, erect and diminished, formed midway between the lens and its focus.

CBSE Class 12 Board · 2003 · Subjective

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