An object placed 20 cm in front of a mirror is found to have an image 15 cm in front of it. Calculate its focal length.

An object placed

20 cm

in front of a mirror is found to have an image

15 cm

in front of it. Its focal length would be

- 8.5 cm .

Given that,
Object distance

u = - 20 cm

Image distance

v = - 15 cm

(negative: as the image is forming in front of the mirror)
Now, applying the Mirror’s formula

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

\Rightarrow \frac{1}{f} = \frac{1}{- 15} + \frac{1}{- 20}

\Rightarrow \frac{1}{f} = \frac{- 4 - 3}{60}

\Rightarrow \frac{1}{f} = \frac{- 7}{60}

\Rightarrow f = \frac{- 60}{7} cm

\Rightarrow f = - 8.5 cm

As the focal length is negative, then the mirror will be Concave mirror.

An object placed $20 cm$ in front of a mirror is found to have an image $15 cm$ in front of it. Calculate its focal length.