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 [tex]v=18cm[/tex].
Now as we know that the mirror formula is,
[tex]\frac{1}{f} =\frac{1}{u} +\frac{1}{v}[/tex]
Now put the values of u and v
[tex]\frac{1}{f} =\frac{1}{\infty } +\frac{1}{18}\\\frac{1}{f}=0+\frac{1}{18}\\f=18 cm[/tex]
Therefore the focal length of the mirror is +18 cm.
