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Basic Properties of Mirrors

Reflection of light is an inherent and important fundamental property of mirrors, and is quantitatively gauged by the ratio between the amount of light reflected from the surface and that incident upon the surface, a term known as reflectivity. Mirrors of different design and construction vary widely in their reflectivity, from nearly 100 percent for highly-polished mirrors coated with metals that reflect visible and infrared wavelengths, to nearly zero for strongly absorbing materials.

The images formed by a mirror are either real or virtual, depending upon the proximity of the object to the mirror, and can be accurately predicted with respect to size and location from calculations based on the geometry of any particular mirror. Real images are formed when the incident and reflected rays intersect in front of the mirror, whereas virtual images occur at points where extensions from incident and reflected rays converge behind the mirror. Planar (flat) mirrors produce virtual images because the focal point, at which extensions from all incident light rays intersect, is positioned behind the reflective surface.

Introduction to Mirrors - In order to reflect light waves with high efficiency, the surface of a mirror must be perfectly smooth over a long range, with imperfections that are much smaller than the wavelength of light being reflected. This requirement applies regardless of the shape of the mirror, which can be irregular or curved, in addition to the planar mirror surfaces commonly seen in households. Curved mirrors are roughly divided into two categories, concave and convex, terms that are also used to describe the geometry of simple thin lenses. With mirrors, the curved surface is referred to as either concave or convex depending upon whether the center of curvature occurs on the side of the reflecting surface or the opposite side.

Interactive Java Tutorials

Concave Spherical Mirrors - Concave mirrors have a curved surface with a center of curvature equidistant from every point on the mirror's surface. An object beyond the center of curvature forms a real and inverted image between the focal point and the center of curvature. This interactive tutorial explores how moving the object farther away from the center of curvature affects the size of the real image formed by the mirror. Also examined in the tutorial are the effects of moving the object closer to the mirror, first between the center of curvature and the focal point, and then between the focal point and the mirror surface (to form a virtual image).

Concave Spherical Mirrors (3-Dimensional Version) - Concave mirrors have a curved surface with a center of curvature equidistant from every point on the mirror's surface. An object beyond the center of curvature forms a real and inverted image between the focal point and the center of curvature. This interactive tutorial explores how moving the object farther away from the center of curvature affects the size of the real image formed by the mirror.

Convex Spherical Mirrors - Regardless of the position of the object reflected by a convex mirror, the image formed is always virtual, upright, and reduced in size. This interactive tutorial explores how moving the object farther away from the mirror's surface affects the size of the virtual image formed behind the mirror.

Convex Spherical Mirrors (3-Dimensional Version) - Regardless of the position of the object reflected by a convex mirror, the image formed is always virtual, upright, and reduced in size. This interactive tutorial explores how moving the object farther away from the mirror's surface affects the size of the virtual image formed behind the mirror. This tutorial utilizes three-dimensional graphics.

Selected Literature References

Reference Listing - This selection of review articles on the basic properties of mirrors reflects their importance in understanding the physics of light and color. The reference section contains book location information for these articles, as well as providing a listing of the appropriate chapter titles dealing with mirror systems.

Contributing Authors

Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.

Matthew J. Parry-Hill, Christopher A. Burdett, Robert T. Sutter, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.


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