A concave mirror of focal length $20$ cm forms a real image at $30$ cm from the pole. The object is at:
A$50$ cm, the simple sum of the two focal values
B$60$ cm, by $1/v + 1/u = 1/f \Rightarrow u = -60$ cm
C$10$ cm, the difference between image and focus
D$20$ cm, equal to the focal length value here
Answer & Solution
Correct answer: B. $60$ cm, by $1/v + 1/u = 1/f \Rightarrow u = -60$ cm
$1/(-30) + 1/u = 1/(-20)$ (signs), $u = -60$ cm; object is 60 cm in front.
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