As suggested
longwinter's writeup, a
spherical mirror will not produce as sharp an image as a
plane mirror will due to the lack of a definite
focal point. However, if the mirror is small relative to its
radius of curvature, the reflected rays will cross at very nearly a single point. This is known as the focal point and the diistance from it along the
principal axis to the mirror is the focal length. The
focal length is half the radius of curvature. However, this only holds if the angle of reflection is small. So,
f = r / 2
The
image formed from a
concave spherical mirror is always
magnified. If the object lies between the focal point and the mirror, then the image is
virtual (behind the mirror) and
upright. Conversely, if the object lies beyond the focal point, the image is
real, and
inverted.
Although not necessarily spherical, a
spoon is an everyday example of this
phenomenon. When you look at the reflection of your face on the inside of a spoon, it's
upside down. This indicates that the focal point lies between your face and the spoon. If you were to move your face closer to the spoon (and the spoon was big enough that you could still see the inside) the image would become non-inverted once your face passed the focal point.
Shaving mirrors which magnify the image are spherical concave reflectors. However the
radius of curvature is large enough that your face is still the right way up when you look in the mirror. If you look at a shaving mirror
from several metres away, the image will be
inverted.