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Optics is the science that describes the behavior and properties of light and the interaction of light with matter. Optics explains optical phenomena. The word optics comes from ὀπτική, meaning appearance or look in ancient Greek).

Optics usually describes the behavior of visible, infrared, and ultraviolet light; however because light is an electromagnetic wave, similar phenomena occur in X-rays, microwaves, radio waves, and other forms of electromagnetic radiation and analogous phenomena occur with charged particle beams. Optics can largely be regarded as a sub-field of electromagnetism. Some optical phenomena depend on the quantum nature of light relating some areas of optics to quantum mechanics. In practice, the vast majority of optical phenomena can be accounted for using the electromagnetic description of light, as described by Maxwell's equations.

The field of optics has its own identity, societies, and conferences. The pure science aspects of the field are often called optical science or optical physics. Applied optical sciences are often called optical engineering. Applications of optical engineering related specifically to illumination systems are called illumination engineering. Each of these disciplines tends to be quite different in its applications, technical skills, focus, and professional affiliations. More recent innovations in optical engineering are often categorized as photonics or optoelectronics. The boundaries between these fields and "optics" are often unclear, and the terms are used differently in different parts of the world and in different areas of industry.

Because of the wide application of the science of "light" to real-world applications, the areas of optical science and optical engineering tend to be very cross-disciplinary. Optical science is a part of many related disciplines including electrical engineering, physics, psychology, medicine (particularly ophthalmology and optometry), and others. Additionally, the most complete description of optical behavior, as known to physics, is unnecessarily complicated for most problems, so particular simplified models are used. These limited models adequately describe subsets of optical phenomena while ignoring behavior irrelevant and/or undetectable to the system of interest.

Classical optics

Before quantum optics became important, optics consisted mainly of the application of classical electromagnetism and its high frequency approximations to light. Classical optics divides into two main branches: geometric optics and physical optics.

Geometric optics, or ray optics, describes light propagation in terms of "rays". Rays are bent at the interface between two dissimilar media, and may be curved in a medium in which the refractive index is a function of position. The "ray" in geometric optics is an abstract object, or "instrument," which is perpendicular to the wavefronts of the actual optical waves. Geometric optics provides rules for propagating these rays through an optical system, which indicates how the actual wavefront will propagate. This is a significant simplification of optics, and fails to account for many important optical effects such as diffraction and polarization. It is a good approximation, however, when the wavelength is very small compared with the size of structures with which the light interacts. Geometric optics can be used to describe the geometrical aspects of imaging, including optical aberrations.

Geometric optics is often simplified even further by making the paraxial approximation, or "small angle approximation." The mathematical behavior then becomes linear, allowing optical components and systems to be described by simple matrices. This leads to the techniques of Gaussian optics and paraxial raytracing, which are used to find first-order properties of optical systems, such as approximate image and object positions and magnifications. Gaussian beam propagation is an expansion of paraxial optics that provides a more accurate model of coherent radiation like laser beams. While still using the paraxial approximation, this technique partially accounts for diffraction, allowing accurate calculations of the rate at which a laser beam expands with distance, and the minimum size to which the beam can be focused. Gaussian beam propagation thus bridges the gap between geometric and physical optics.

Physical optics or wave optics builds on Huygens's principle and models the propagation of complex wavefronts through optical systems, including both the amplitude and the phase of the wave. This technique, which is usually applied numerically on a computer, can account for diffraction, interference, and polarization effects, as well as other complex effects. Approximations are still generally used, however, so this is not a full electromagnetic wave theory model of the propagation of light. Such a full model is much more computationally demanding, but can be used to solve small-scale problems that require this more accurate treatment.

Everyday optics

Optics is part of everyday life. Rainbows and mirages are examples of optical phenomena. Many people benefit from eyeglasses or contact lenses, and optics are used in many consumer goods including cameras. Superimposition of periodic structures, for example transparent tissues with a grid structure, produces shapes known as moiré patterns. Superimposition of periodic transparent patterns comprising parallel opaque lines or curves produces line moiré patterns.


  • Optics (4th ed.) Pearson Education, ISBN 0-8053-8566-5}}
  • Physics for Scientists and Engineers (6th ed.) ISBN 0-534-40842-7
  • Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics, W. H. Freeman, ISBN 0-7167-0810-8
  • Optical Physics (3rd ed.), Cambridge University Press, ISBN 0-5214-3631-1

Textbooks and tutorials

  • Optics — an open-source Optics textbook
  • Optics2001 — Optics library and community