Question

A spherical mirror produces an image has size 5 times greater than the object on a screen, 5 meters away from the object. The mirror is...
A. concave with the focal length of 25/24 m
B. convex with the focal length of 25/24 m
C. concave with the focal length of 24/25 m
D. convex with the focal length of 24/25 m​

Answer 1

$\boxed{\bf{\blue{\longrightarrow Answer}}}$

• As image is enlarged it is concave mirror
• Given, v-u=5
• Also magnification= 5
• $\frac{ - v}{u} = 5$
• $v = - 5u$
• Replace this value
• -5u-u=5
• $u = \frac{ - 5}{6}$
• $v = 5 ×- (\frac{ - 5}{6} )$
• $v = \frac{25}{6}$
• Using mirror formula
• $\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$
• $\frac{1}{f} = \frac{6}{25} - \frac{6}{5}$
• $\frac{1}{f} = \frac{ - 24}{25}$
• $f = \frac{-25}{24}$
• So option A is correct