A flat mirror

We can even apply the equation to a flat mirror. As a sphere gets bigger and bigger, its surface is more and more gently curved. The planet Earth is so large, for example, that we cannot even perceive the curvature of its surface. To represent a flat mirror, we let the mirror's radius of curvature, and its focal length, become infinite. Dividing by infinity gives zero, so we have

1/d_o = -1/d_i ,

or

d_o = -d_i .

If we interpret the minus sign as indicating a virtual image on the far side of the mirror from the object, this makes sense.