Question

If an object is placed 25.0 cm from a concave mirror whose focal length is 5.0 cm, where is the image located?

Answer 1

For this question, we need to use the mirror formula `1/f= 1/d_o +1/d_i`.

What the problem gives us is: f = 5.0 cm, and `d_o` = 25.0 cm. So we are solving for `d_i`. Isolating the unknown to its own side of the equation, in this case by subtracting `1/d_i` from both sides, will accomplish this.

`1/d_i = 1/f − 1/d_o`

`1/d_i = 1/5 − 1/25`. FInd a common denominator.

`1/d_i = 5/25 − 1/25`

`1/d_i = 4/25`. To find `d_i`, take the reciprocal.

`d_i = 25/4 = +6.25 cm`

We know that this is a real image because `d_i` is positive.

The same process can be used if you know the distance from the image to the vertex of the mirror, and are looking for `d_o`.