Question

A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 cm in such away that the end closer to the pole is 20 cm away from it. Find the length of the image.

• A. 2 cm
• B. 4 cm
• C. 5 cm
• D. 6 cm

Answer 1

Answer: (C)

5 cm

The position of image of nearer end to mirror can be found out using mirror equation,

$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$

$⟹\frac{1}{v_1}=\frac{1}{-20}-\frac{1}{-10}$

$⟹v_2=-20$

Similarly position of image of farther end to mirror can be found out as

$=\frac{1}{v_2}=\frac{1}{-(10+20)}-\frac{1}{-10}$

$⟹v_2=-15$

Hence the length of the image $=20cm-15cm=5cm$