CD-R is a Write Once Read Many ( WORM ) optical medium, although the whole disk does not have to be entirely written in the same session.

A laser consists of a gain medium, a mechanism to supply energy to it, and something to provide optical feedback.

The most common type of laser uses feedback from an optical cavity — a pair of mirrors on either end of the gain medium.

The beam in the cavity and the output beam of the laser, when travelling in free space ( or a homogenous medium ) rather than waveguides ( as in an optical fiber laser ), can be approximated as a Gaussian beam in most lasers ; such beams exhibit the minimum divergence for a given diameter.

The lasing medium in some dye lasers and vibronic solid-state lasers produces optical gain over a wide bandwidth, making a laser possible which can thus generate pulses of light as short as a few femtoseconds ( 10 < sup >− 15 </ sup > s ).

The gain medium is excited by the pump source to produce a population inversion, and it is in the gain medium that spontaneous and stimulated emission of photons takes place, leading to the phenomenon of optical gain, or amplification.

The optical resonator, or optical cavity, in its simplest form is two parallel mirrors placed around the gain medium which provide feedback of the light.

Some lasers do not use an optical cavity, but instead rely on very high optical gain to produce significant amplified spontaneous emission ( ASE ) without needing feedback of the light back into the gain medium.

The wave model can be used to make predictions about how an optical system will behave without requiring an explanation of what is " waving " in what medium.

These photons are fed back into the laser medium, usually by an optical resonator.

In particular, the pump band may consist of several distinct energy levels, or a continuum of levels, which allow optical pumping of the medium over a wide range of wavelengths.

In optics the refractive index ( or index of refraction ) n of a substance ( optical medium ) is a number that describes how light, or any other radiation, propagates through that medium.

Such a gain medium, along with an optical resonator, is at the heart of a laser or maser.

Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.

The optical depth expresses the quantity of light removed from a beam by scattering or absorption during its path through a medium.

Note that the optical depth of a given medium will be different for different colors ( wavelengths ) of light.

The active laser medium ( also called gain medium or lasing medium ) is the source of optical gain within a laser.

In optical systems, beam steering may be accomplished by changing the refractive index of the medium through which the beam is transmitted or by the use of mirrors, prisms, lenses, or rotating diffraction gratings.

In fiber optics, a cladding mode is a mode that is confined to the cladding of an optical fiber by virtue of the fact that the cladding has a higher refractive index than the surrounding medium, which is either air or the primary polymer overcoat.

The concept of the strain energy as a Gibbs function difference Af and exerting a force normal to the shearing face is compatible with the information obtained from optical birefringence studies of fluids undergoing shear.

Flatness may now monopolize everything, but it is a flatness become so ambiguous and expanded as to turn into illusion itself -- at least an optical if not, properly speaking, a pictorial illusion.

) An authentic diffraction pattern is always obtained and optical properties are frequently checked.

It should be noted, however, that the paraxial resolution is quite similar for both electron optical systems.

The electron optical system ( see fig. 14-1 ) is based in principle on the focusing action of concentric spherical cathode and anode surfaces.

It is clear from this discussion that cosmologists of every persuasion look hopefully toward the day when a man-made satellite can be equipped with optical devices which will open up new vistas to science.

As with many scientific fields, strict delineation can be highly contrived and atomic physics is often considered in the wider context of atomic, molecular, and optical physics.

The and convention for amino acid configuration refers not to the optical activity of the amino acid itself, but rather to the optical activity of the isomer of glyceraldehyde from which that amino acid can, in theory, be synthesized (- glyceraldehyde is dextrorotary ;-glyceraldehyde is levorotatory ).

Alexander's band, an optical phenomenon, is named after him.

Shot noise is a type of electronic noise that occurs when the finite number of particles ( such as electrons in an electronic circuit or photons in an optical device ) is small enough to give rise to statistical fluctuations in a signal.

Atomic absorption spectroscopy ( AAS ) is a spectroanalytical procedure for the quantitative determination of chemical elements employing the absorption of optical radiation ( light ) by free atoms in the gaseous state.

An optical aberration is a departure of the performance of an optical system from the predictions of paraxial optics.

Aberrations occur because the simple paraxial theory is not a completely accurate model of the effect of an optical system on light, rather than due to flaws in the optical elements.

Defocus is the lowest-order true optical aberration.

The elementary theory of optical systems leads to the theorem: Rays of light proceeding from any object point unite in an image point ; and therefore an object space is reproduced in an image space.

The Gaussian theory, however, is only true so long as the angles made by all rays with the optical axis ( the symmetrical axis of the system ) are infinitely small, i. e. with infinitesimal objects, images and lenses ; in practice these conditions are not realized, and the images projected by uncorrected systems are, in general, ill defined and often completely blurred, if the aperture or field of view exceeds certain limits.

Math., 1858 ) and Ernst Abbe showed that the properties of these reproductions, i. e. the relative position and magnitude of the images, are not special properties of optical systems, but necessary consequences of the supposition ( in Abbe ) of the reproduction of all points of a space in image points ( Maxwell assumes a less general hypothesis ), and are independent of the manner in which the reproduction is effected.

In pinhole projection, the magnification of an object is inversely proportional to its distance to the camera along the optical axis so that a camera pointing directly at a flat surface reproduces that flat surface.

* One application of autocorrelation is the measurement of optical spectra and the measurement of very-short-duration light pulses produced by lasers, both using optical autocorrelators.

Optical infinity — This was achieved by adjusting the focal distance so that it was above what is sometimes referred to as " optical infinity ", which is generally taken as about 30 ft or nine m. In this context, " Optical Infinity " is the distance at which, for the average adult person, the angle of view of an object at that distance is effectively the same for both the left and right eyes.

For example a 1 meter diameter optical telescope is two million times bigger than the wavelength of light observed giving it a resolution of roughly 0. 3 arc seconds, whereas a radio telescope " dish " many times that size may, depending on the wavelength observed, only be able to resolve an object the size of the full moon ( 30 minutes of arc ).

Also, in 1931 the US Army worked on a mobile antiaircraft machine mount on the back of a heavy truck having four. 30 caliber water-cooled machine guns and an optical director.

OSA was founded in 1916, under the leadership of Perley G. Nutting, with 30 optical scientists and instrument makers based in Rochester, New York.

Some advanced techniques ( involving high harmonic generation with amplified femtosecond laser pulses ) can be used to produce optical features with durations as short as 100 attoseconds in the extreme ultraviolet spectral region ( i. e. < 30 nm ).

On 30 October 2006, researchers from the University of Connecticut demonstrated new vulnerabilities in Diebold AccuVote-OS optical scan voting terminal.

The MSS FPA, or Focal Plane Array consisted of 24 square optical fibers extruded down to. 0002 " square fiber tips in a 4x6 array to be scanned across the Nimbus spacecraft path in a +/- 6 degree scan as the satellite was in a 10: 30 polar orbit, hence it had to be launched from Vandenberg AFB.

These new developments allow the optical resolution to approach ~ 30 nm.

The normal optical depths of the arcs are estimated to lie in the range 0. 03 – 0. 09 ( 0. 034 ± 0. 005 for the leading edge of Liberté arc as measured by stellar occultation ); the radial widths are approximately the same as those of the continuous ring — about 30 km.

A total of two million images comprising seventy terabytes, and its Blackletter type was scanned using optical character recognition at a total cost of 600, 000 € ( or 0. 30 € per image ).

For example, some regions of the Galactic center have more than 30 magnitudes of extinction in the optical, meaning that less than 1 optical photon in 10 < sup > 12 </ sup > passes through.

The optical pulsar is roughly 25 km in diameter and the pulsar " beams " rotate once every 33 milliseconds, or 30 times each second.

Laserdiscs, the first consumer optical discs, used constant linear velocity to double playback time ( CLV disc can hold 1 hour per side ; CAV disc only 30 minutes ).

Caslon Old Face is a typeface with multiple optical sizes, including 8, 10, 12, 14, 18, 22, 24, 30, 36, 42, 48, 60, 72, 96 points.

All the optical elements, including the photomask, must make use of defect-free Mo / Si multilayers which act to reflect light by means of interlayer interference ; any one of these mirrors will absorb around 30 % of the incident light.

In a recent actual EUV print test, it was found 30 nm spaces could not be resolved, even though the optical resolution and the photoresist composition were not the limiting factor.