Question

A short linear object, of length $l$, lies along the axis of a concave mirror, of focal length $f$, at a distance $d$ from the pole of the mirror. The size of the image is then (nearly)

• A. $\frac{lf}{d-f}$
• B. $\frac{d-f}{lf}$
• C. $l\frac{f^{ 2}}{\left(d-f\right)^{2}}$
• D. $\frac{\left(d-f\right)^{2}}{f^{ 2}}l$

Answer 1

Answer: (C)

$l\frac{f^{ 2}}{\left(d-f\right)^{2}}$

Answer 2

Use $\frac{1}{\upsilon}+\frac{1}{u}=\frac{1}{f}$ Find $\upsilon_{1},$ when $u = d$ and $\upsilon_{2},$ when $u = d + l$ Size of image $=\upsilon_{1}-\upsilon_{2}=l\left(\frac{f}{d-f}\right)^{2}$