On an optical bench of 150 cm, a student used a concave mirror of focal length 20 cm to observe object distance (u) and image distance (v) respectively as:
(A) (40 cm, 10 cm)
(B) (60 cm, 15 cm)
(C) (80 cm, 38 cm)
(D) (60 cm, 30 cm)
(E) (40 cm, 40 cm)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (D) and (E) only
To check which of the given object (u) and image (v) distances correspond to the concave mirror of focal length (f) = 20 cm
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$ [lens formula]
(D) $u=60cm,v=30cm$
$\frac{1}{f}=\frac{1}{30}+\frac{1}{60}$
$\frac{1}{f}=\frac{60}{3}=20cm$
(E) $u=40cm,v=40cm$
$\frac{1}{f}=\frac{2}{40}=20cm$