#### Term Decibel (dB)

Term definition

A logarithmic unit of the ratio of a quantity, usually power or intensity levels relative to a specified level. For example, when used to describe the gain or loss in power it is defined as:

$\text{Level in dB}=10\text{\hspace{0.17em}}\mathrm{log}\frac{\text{power level 2}}{\text{power level 1}}$

The level in decibels is positive if power level 2 is greater than power level 1, negative if it is less, and zero if they are equal.

Term etymology

early 1900s: deci- [ten] + bel (the unit being one tenth of a bel, named after Alexander Graham Bell).

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#### Term Deflect

Term definition

To cause something to change direction or travel away from its expected path.

For more about deflection click on this link to the pamphlet on Lasers.

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#### Term Detector

Term definition

1) A device designed to detect the presence of something and provide a signal in response. 2) A device designed to convert a range of incident energy of radiation into another form to determine the presence and/or amount of radiation. The device may function by electrical, photographic, or visual means.

Term etymology

1447, from L. detectus, pp. of detegere "uncover, disclose," from de- "un-, off" + tegere "to cover."

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#### Term Diffraction

Term definition

1) A phenomenon that occurs whenever a light wave is obstructed in any way. Often diffraction fringes can be seen when a small aperture or object blocks light waves. 2) The optical phenomenon by which a grating separates the light into its constituent components. Diffraction causes different wavelengths of light to transmit through or reflect from a grating at different angles, allowing a spectrometer to separate and measure intensity of individual wavelengths.

Term etymology

1671, from Fr. diffraction, from Mod.L. diffractionem, from L. diffrac-, stem of diffringere "break in pieces," from dis- "apart" + frangere "to break."

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#### Term Diffraction grating

Term definition

A device used to break light into its component wavelengths. It is usually composed of a material with tiny grooves cut into it. These disperse the light as it passes through or bounces off the grating (depending on the type of grating). Physicists and astronomers often use diffraction gratings to determine the wavelengths composing the light being viewed.

Term etymology

c.1374, from L. diffusionem, from stem of diffundere "scatter, pour out," from dif- "apart, in every direction" + fundere "pour."

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#### Term Diffuse Reflection

Term definition

A phenomenon that results when light strikes an irregular surface such as a frosted window or the surface of a frosted or coated light bulb.

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#### Term Diffusion

Term definition

1) Is a transport phenomena that results in mixing due to random motion of atoms and molecules. 2) In optics, the scattering of light due to reflection or transmission.

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#### Term Digital camera

Term definition

A camera that records images in digital form by converting the light from the scene being photographed into an electric signal with the use of charge-coupled devices (CCDs). The electric signal is then stored digitally on a random access memory device. The digital data may then be manipulated to enhance or otherwise modify the resulting viewed image.

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#### Term Diopter (D)

Term definition

1) A unit of optical power that expresses the refractive power of a lens or a mirror. For a lens, it is equal to the reciprocal of the focal length, in meters. For example, a 5 diopter lens brings parallel rays of light shining on it to a focus 1/5 of a meter away from its center of curvature. 2) A prism diopter (∆) is a measure of prismatic deviation equal to a deflection of 1 cm at a distance of 1 m.

Term etymology

From L. dioptra, from Gk. dioptra : dia- + optos, "visible." A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters.

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#### Term Dioptric system

Term definition

An optical system that uses lenses for image formation.

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#### Term Direct ray

Term definition

A ray that travels without being reflected or refracted.

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#### Term Direct-vision prism

Term definition

An assembly of multiple prisms that disperses incident light into its spectral components without deviating light at the central wavelength.

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#### Term Dispersing prism

Term definition

A prism or series of prisms used to disperse a beam of radiant energy of mixed wavelengths into its spectral components.

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#### Term Dispersion

Term definition

The dependence of a wave’s velocity on its frequency. Objects that have this property are called dispersive media, and can separate a beam of light into its various wavelength components, such as a dispersive prism. Another common example of light dispersion is a rainbow.

Term etymology

c.1450, from M.Fr. disperser "scatter," from L. dispersus, pp. of dispergere "to scatter," from dis- "apart, in every direction" + spargere "to scatter."

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#### Term Distortion

Term definition

The situation where an image is not a true-to-scale reproduction of an object.

Term etymology

1586, from L. distortus, pp. of distorquere "to twist different ways, distort," from dis- "completely" + torquere "to twist."

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#### Term Diverge

Term definition

To separate, or cause to separate, and go in different directions from a point.

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#### Term Divergence

Term definition

The bending of rays away from each other.

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#### Term Diverging lens

Term definition

A lens that causes parallel rays of light to spread out. Examples include: negative lens, divergent lens, concave lens, or dispersive lens.

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#### Term Doppler effect

Term definition

Radiation emitted from a source and received by an observer that are in relative motion to each other appears to be of a lower or higher frequency than if there were no relative motion between source and observer. If the relative motion between source and receiver causes motion toward each other, the frequency is shifted upward (sometimes called blue shifted). If the source and receiver are moving away from each other the frequency is shifted downward (sometimes termed red shifted).

Term etymology

1871, in reference to Christian Doppler (1803-53), Austrian scientist, who in 1842 explained the effect of relative motion on waves (originally to explain color changes in binary stars); proved by musicians performing on a moving train. Doppler shift is the change of frequency resulting from the Doppler effect.

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#### Term Doublet lens

Term definition

A compound lens consisting of two elements.

Term etymology

c.1225, from O.Fr. duble, from L. duplus "twofold," from duo "two" + -plus "fold."

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#### Term Dove prism

Term definition

A form of prism invented by H.W. Dove. It resembles half of a common right-angle prism in which a ray entering parallel to the hypotenuse face is reflected internally at that face and emerges parallel to its incident direction. One of the incident rays emerges along a continuation of its incident direction, and if the prism is rotated about that ray through some angle, the image rotates through twice that angle. A Dove prism must be used in parallel light.

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