Find focal length of concave mirror(Numerical Question) solved

 

Numerical Question of Chapter 10 Light Class 10 Science 

An object is placed at a distance of 30 cm in front of a concave mirror. Its image is formed at a distance of 20 cm. If the image is real, find the focal length of the mirror.

Given values

Object distance(u) = -30 cm

Image distance (v) = - 20 cm

[as the image formed is real so image distance has been taken as negative]

Focal length(f) =?

Solution

`1/u+  1/v=1/f` 

`1/f=  1/v+  1/u` 

`1/f=  1/(-20)+  1/(-30)` 

`1/f=(-3 -2 )/60` 

`1/f=(-5)/60` 

`f=  60/(-5)` 

`f= -12 cm` 

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1. Numerical Quesitons