For a flat mirror the object and image distances, p and q, are the same, and the object and image sizes are the same, so the magnification is 1.0.

For the concave mirror, the focal distance f = R/2 and the object and image distances are related by (1/p) + (1/q) = (1/f).

Sign rule: p is positive if the object is in front, q is positive if the image is in front, and f is positive if the focus is in front.

Because light is actually brought to a focus by a concave mirror, the image it forms is said to be "real," unlike the image formed by a flat mirror. The image seen in a flat mirror is called "virtual," because the light seen does not come from the point in space the image appears to occupy.