Image-forming optical system
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In optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

, an image-forming optical system is a system capable of being used for imaging
Image
An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...

. The diameter of the aperture of the main objective is a common criteria for comparison among optical systems, such as large telescopes.

The two traditional systems are mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

-systems (catoptrics
Catoptrics
Catoptrics deals with the phenomena of reflected light and image-forming optical systems using mirrors. From the Greek κατοπτρικός ....

) and lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

-systems (dioptrics
Dioptrics
Dioptrics is the study of the refraction of light, especially by lenses. Telescopes that create their image with an objective that is a convex lens are said to be "dioptric" telescopes....

), although in the late twentieth century, optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

 was introduced. Catoptrics and dioptrics have a focal point
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

, while optical fiber transfers an image from one plane to another without an optical focus.

Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 is reported to have designed what he called a catadioptric
Catadioptric
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses and curved mirrors . Catadioptric combinations are used in focusing systems such as search lights, headlamps, early lighthouse focusing systems, optical telescopes,...

al phantasmagoria
, which can be interpreted to mean an elaborate structure of both mirrors and lenses.

Catoptrics and optical fiber have no chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

, while dioptrics need to have this error corrected. Isaac Newton believed that such correction was impossible, because he thought the path of the light depended only on its color. In 1757 John Dollond
John Dollond
John Dollond was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets.-Biography:...

 was able to create an achromatised dioptric, which was the forerunner of the lenses
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...

 used in all popular photographic equipment today.

Lower-energy X-Rays are the highest energy electromagnetic radiation that can be formed into an image, using a Wolter telescope
Wolter telescope
A Wolter telescope is a telescope for X-rays using only grazing incidence optics. Visible light telescopes are built with lenses or parabolic mirrors at nearly normal incidence. Neither works well for X-rays. Lenses for visible light are made of a transparent material with an index of refraction...

. There are three types of Wolter telescopes Near infrared is typically the longest wavelength that are handled optically, such as in some large telescopes.
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