Question

An object is kept before the mirror at the distance of 10cm and its image is created at the distance of 30cm. What will be it's focal length?
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Answer 1

Answer:

7.5cm

Explanation:

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

(+ as it is a mirror)

$\frac{1}{f} = \frac{1}{ - 30} + \frac{1}{ - 10}$

$\frac{1}{f} = \frac{ - 1}{30} - \frac{1}{10}$

$\frac{1}{f} = \frac{ - 1 - 3}{30}$

$\frac{1}{f} = \frac{ - 4}{30}$

$f = \frac{ - 15}{2} = 7.5cm$