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

1 Answer
Apr 30, 2014

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.