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# An object placed 18cm in front of a convex mirror produced an image 6cm behind t...

### Question

An object placed 18cm in front of a convex mirror produced an image 6cm behind the mirror. Find the focal length of the mirror.

A) 9.0cm

B) 7.5cm

C) -6.5cm

D) -9.0cm

E) 4.5cm

### Explanation:

By mirror formula:
$$\cfrac{1}{f} = \cfrac{1}{u} + \cfrac{1}{v} = \cfrac{1}{18} + \cfrac{1}{6} = \cfrac{1+3}{18} = \cfrac{4}{18}$$
$$f = \cfrac{18}{4}$$
= 4.5cm

## Dicussion (1)

• By mirror formula:
$$\cfrac{1}{f} = \cfrac{1}{u} + \cfrac{1}{v} = \cfrac{1}{18} + \cfrac{1}{6} = \cfrac{1+3}{18} = \cfrac{4}{18}$$
$$f = \cfrac{18}{4}$$
= 4.5cm