Question

If the magnification of image is -0.8 and height of object is 5cm, find focal length ,and which type of mirror it forms on??

Answer 1

Explanation:

$m \: = \frac{ - v}{u} \\ = > - 0.8 \: = \frac{ - v}{5 } \\ v \: = 4 \: cm \\ \frac{1}{f} = \: \frac{1}{v } \: + \: \frac{1}{u} \\ = > \: \frac{1}{f} = \frac{1}{4} \: - \frac{1}{5} \\ = > \: \frac{1}{f} = \: \frac{1}{20} \\ = > f \: = \: 20 \: cm$

Since the focal length is + 20 cm, the image is formed on a convex mirror as the focal length of a convex mirror is always positive and that of a concave mirror is always negative.