A concave mirror of focal length $20$ cm forms a real image of an object placed $30$ cm from the mirror. Find the image distance from the mirror.
A$12$ cm
B$15$ cm
C$30$ cm
D$60$ cm
Answer & Solution
Correct answer: D. $60$ cm
Mirror formula: $\dfrac{1}{f} = \dfrac{1}{v} + \dfrac{1}{u}$. With the standard convention $f = -20$ cm and $u = -30$ cm:
$\dfrac{1}{-20} = \dfrac{1}{v} + \dfrac{1}{-30} \Rightarrow \dfrac{1}{v} = -\dfrac{1}{20} + \dfrac{1}{30} = -\dfrac{1}{60}$
So $v = -60$ cm, i.e. the image is $60$ cm in front of the mirror.
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