Convex mirror: A curved mirror whose
reflective side is on the outside of the curve.
Mirrors can be curved as well as flat. One type of curved mirror is a spherical mirror,
which consists of a section of a sphere. To describe spherical mirrors it is common to
consider a sphere with silvered reflecting surfaces. In a concave mirror, reflection
occurs on the inside surface, as illustrated in Concept 2. A convex mirror is made by
using a section of the exterior of the sphere, as shown in Concept 3.
If you like mnemonics, a good way to remember which type is which is to recall that a
concave mirror is like a cave whose walls curve around you. Though we concentrate on
spherical mirrors here, concave and convex are terms that can apply to curved mirrors
other than spherical mirrors.
The photos above show two examples of spherical mirrors and the images they create.
The left-hand photo shows a concave makeup mirror. These mirrors, often found in
bathrooms, are designed to magnify an image: You look in the mirror and see a larger
image of yourself. Lucky you!
The photograph on the right above shows a convex safety mirror that allows drivers to
see around a corner of a twisty seaside road. Security mirrors are also often convex.
They provide a larger field of view than a flat mirror, enabling store personnel, for
example, to survey a large area. Convex auxiliary mirrors are often affixed to the flat
rearview mirrors of trucks and RVs.
Spherical mirror
A portion of a sphere
Concave mirrors
Reflecting surface is interior of sphere
Convex mirrors
Reflecting surface is exterior of sphere
31.8 - Mirror terminology
Before we proceed to a further discussion of mirrors, we need to introduce some
terminology. The diagrams to the right are used to illustrate these new terms. Many of
the same terms are used to describe lenses, a topic you will study later.
The mirrors discussed in this section are spherical. The sphere’s center of curvature is
shown as point C in the diagrams.
A line passing through the center of curvature and the midpoint of a spherical mirror,
like the one shown in Concept 1, is called the principal axis of the mirror. This line is
perpendicular to the surface of the mirror where they intersect.
While planar mirrors produce only virtual images, a curved mirror can produce either a
virtual or a real image, depending on the shape of the mirror and the location of the
object.
The image point corresponding to an object on the principal axis is the location on the
axis of its image. When an object is infinitely far away from the mirror, its image point
defines the focal point of a mirror or lens. You see this point, F, in Concept 2. “Infinitely
far away” means the incident light rays from the object are effectively parallel. The focal
point is located on the principal axis, and it is an unchanging characteristic of a
particular mirror.
The distance between the focal point and the midpoint of a mirror is its focal length, represented with the symbolf. By convention, concave
mirrors have positive focal lengths, while convex mirrors have negative ones. You see an example of a negative focal length in Concept 3. This
Mirror terminology
Center of curvature C is center of
sphere
Principal axis passes through center of
curvature and midpoint of mirror