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Depth of field
Air
Air
great) at about 135dB. Each 3dB increase in sound represents a doubling of sound intensity. (See also: Sound level.)
Decay
The gradual reduction in the size of a sound wave so that the sound fades away and finally stops. Decay can be speeded
up by using damping. In many musical instruments decay is slow, allowing the instrument to give the full range of its sound. (See also: Reverberation.)
Depth of field
The range of distances over which a photograph will be sharp, or in focus. The depth of field depends on how close the camera is to the object, the type of lens and the size of the aperture used. In general, the wider the angle of the lens, the smaller the aperture (f-number). The farther away the scene, the greater the depth of field. A 50mm lens on a 35mm camera set to focus on 10m will have a depth of field from about 4 metres to infinity.
Glass
Glass
Critical angle
Critical angle
Short for the critical angle of total internal reflection. It is the angle at which a transparent material stops allowing light to pass through it and begins to reflect it instead. To understand this, imagine looking directly at a sheet of glass. It will be completely transparent. Now turn the glass. At a certain angle you will no longer be able to see through it, but the glass will behave like a mirror instead. That is the critical angle.
D
Damping
Making a vibration smaller in order to stop reverberation and act as soundproofing. (See also: Decay.)
Decibel
A scale developed to describe the intensity (power) of sound.
The decibel scale is similar to the Richter scale used for
 Depth of field – The depth of field depends on the focal length of the lens and the aperture. If the aperture is large, the depth of field is smaller. In the left-hand picture the focus is on the house – the flowers in the foreground are out of focus because the depth of field doesn’t reach that close. In the right-hand picture the focus is on the flowers, and the house is out
of focus.
 Critical angle – Light passing through a glass block will go back out into the air unless it strikes at a wide angle. Then it will be reflected. The critical angle is the maximum angle for refraction.
earthquakes and is designed to take in a huge range of energy levels. The term ‘bel’ is taken from the name of Alexander Graham Bell, inventor of the telephone. A decibel (dB) is one-tenth of a bel.
On the decibel scale the smallest sound that can be heard is 0dB. Sound becomes painful (because the pressure on the ear is too
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