You're probably familiar with a flat mirror, the most common household type of mirror. You see one of these every time you look into your bathroom mirror, or any time you notice your reflection in a window. These mirrors reflect light so the image you observe is exactly the same size as the object you are observing. The two other most common types of mirrors are the ones you ask about: convex and concave mirrors. Instead of a flat piece of glass, imagine a large, round bowl. Looking at the outside of the bowl, you would see the surface curving away in all directions. This is the idea behind a convex mirror. A convex mirror bends light as it reflects the light, and the farther away a point is from the center, the more the light is bent. As a result, an image formed in a convex mirror is smaller than an image in a plane (flat) mirror. Because the image is smaller, more image can fit onto the mirror, so a convex mirror provides for a larger field of view than a plane mirror. This is why they are useful. They are used whenever a mirror with a large field of view is needed. For example, the passenger-side rear view mirror on a car is convex. You may have noticed that many of these mirrors say "Objects in mirror are closer than they appear." This is because the objects are made smaller by the mirror, so they appear farther away! These mirrors are also used for security in buildings near corners, enabling people to see around a corner before they actually reach it.
The other kind of mirror you ask about is a concave mirror. This is the opposite side of the glass bowl from earlier; a concave mirror bends light inwards, towards you. This mirror is a little trickier. If you imagine the mirror as a part of a larger sphere, then if you view the mirror from a point closer than the center of this sphere, you see an enlarged image. Look further away from the center and you see a diminished image. These mirrors are generally used in everyday life with a pretty small curvature, so that you are always using it in its "magnification" area and you observe an enlarged image. This kind of mirror is used whenever you want to make an image larger! Makeup mirrors and dental mirrors are concave mirrors.
One other kind of mirror that you may not have heard of before is a "parabolic reflector." Instead of a sphere, this mirror is shaped like a paraboloid (think about a satellite dish). Paraboloids have the unique property that any parallel incoming rays of light all bounce off the mirror and meet at exactly one point, called the focus. These are used whenever you want to take incoming parallel light and focus it together (such as on a telescope or a satellite dish). You can also use them to take light in the opposite direction, starting from a single point and projecting it along a straight line, as with a spotlight or with the headlights of a car.
has some nice pictures of convex and concave mirrors, as well as illustrations to explain how they work. Check them out at
Thanks for the question!
(published on 02/09/2011)