Question

A concave mirror forms the image of a very distant object at 18.0 cm on the same side of its reflecting surface. What is the focal length of this mirror? (A) + 36.0 cm (B) + 18.0 cm (C) - 18.0 cm (D) - 36.0 cm

Answer 1

Answer:

(B) +18 cm

Explanation:

Given that the distance of the object is infinity.

And the image is formed at a distance of 18 cm, means $v=18cm$.

Now as we know that the mirror formula is,

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

Now put the values of u and v

$\frac{1}{f} =\frac{1}{\infty } +\frac{1}{18}\\\frac{1}{f}=0+\frac{1}{18}\\f=18 cm$

Therefore the focal length of the mirror is +18 cm.