Take a shiny spoon and try to find your image in it. Now, turn it over. Did you notice that the images produced are very different depending on whether you are looking at a curved-in surface or the curved-out surface? You probably noticed that either way, the image was distorted. That's because the light rays are not bouncing off of a flat surface, but a curved one. Curved mirrors serve a very specific purpose.
For now, we will focus on purely spherical mirrors. A spherical mirror is a mirror that has a spherical shape. In other words, it is a slice off of a sphere. If it was extended in all directions, it would create a sphere with a center point called its center of curvature (C). If you were to draw a line from the center of the mirror to the center of curvature, that would become the principal axis of the mirror. The point where the principal axis meets the mirror is called the vertex (A). The radius of curvature (R) is the distance from the vertex to the center of curvature along that principal axis. Halfway between the center of curvature and the mirror is a point called the focal point (F). The focal length (f) is the distance from the focal point to the vertex.
There are two types of spherical mirrors: concave and convex. Look through each of the tabs below to learn specifically about concave and convex mirrors.
Concave Mirror
Convex Mirror
A concave mirror is also known as a converging mirror. That's because all of the light rays that come into the mirror parallel to the principal axis converge on the focal point. In fact, that's one of the two rules of reflection for a concave mirror. The reflective surface on a concave mirror would be on the inside of the sphere.
The image location and size can be predicted using ray diagrams. You can use the two rules for concave mirrors to help you draw your ray diagrams.
- If an incident ray travels parallel to the principal axis on the way to the concave mirror, the ray will be reflected back through the focal point.
- If an incident ray travels through the focal point on the way to the concave mirror, the ray will be reflected back parallel to the principal axis.
A convex mirror is similar to a concave mirror, but the reflected surface is on the outisde of the sphere. It is also known as a diverging mirror because all of the light rays that come into the reflecting surface bounce away from the focal point that is behind the mirror.
The location and size of the image can be predicted by ray diagrams using two rules, similar to those of the concave mirror. There are some differences in the rules, though.
- If an incident ray travels parallel to the principal axis on the way to the convex mirror, the ray will be reflected in away from the focal point behind the mirror. (If you extend the reflected ray behind the mirror, the extension will pass through the focal point.)
- If an incident ray travels toward the focal point on the way to the convex mirror, the ray will be reflected back parallel to the principal axis. (If the incident ray were to pass through the mirror, the ray would pass through the focal point behind the mirror.)
