Decibel (dB)

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:

Level in dB=10log power level 2 power level 1 MathType@MTEF@5@5@+=feaagCart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqipu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeiiaiaabYeacaqGLbGaaeODaiaabwgacaqGSbGaaeiiaiaabMgacaqGUbGaaeiiaiaabsgacaqGcbGaeyypa0JaaGymaiaaicdacaaMc8UaciiBaiaac+gacaGGNbWaaSaaaeaacaqGWbGaae4BaiaabEhacaqGLbGaaeOCaiaabccacaqGSbGaaeyzaiaabAhacaqGLbGaaeiBaiaabccacaqGYaaabaGaaeiCaiaab+gacaqG3bGaaeyzaiaabkhacaqGGaGaaeiBaiaabwgacaqG2bGaaeyzaiaabYgacaqGGaGaaeymaaaaaaa@5D6E@

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.

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


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.


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.

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


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.

To learn more about diffraction click on the link to the pamphlet about Solid-State Lighting (LEDs) and on the Laser pamphlet.

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

Diffraction grating

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.

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

Diffuse Reflection

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.


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.

Digital camera

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.

Diopter (D)

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.

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.

Dioptric system

An optical system that uses lenses for image formation.

Direct ray

A ray that travels without being reflected or refracted.

Direct-vision prism

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

Dispersing prism

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


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.

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


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

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


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

For more information on divergence, click the link to the pamphlet on Lasers.


The bending of rays away from each other.

Diverging lens

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

Doppler effect

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).

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.

Doublet lens

A compound lens consisting of two elements.

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

Dove prism

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.