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

(B) +18 cm

Step-by-step 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,

`(1)/(f) =(1)/(u) +(1)/(v)`

Now put the values of u and v

`(1)/(f) =(1)/(\infty ) +(1)/(18)\\(1)/(f)=0+(1)/(18)\\f=18 cm`

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