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Diffraction of light
Diffraction of light
The bending of light when it passes around the edge of an object rather than through it (which is called refraction). The effect was first described in 1818 by the French physicist Augustin-Jean Fresnel. It happens where the wavelength of the light is large in comparison with the object. The wave front bends around the object, and there is no shadow. To study this effect, a bright light is shone through glass etched with thousands of parallel lines per centimetre.
Diffraction of sound
The bending of sound waves around an object that occurs
when the waves are long compared with the size of the object.
Because sound waves have long wavelengths – often more than a metre – sound commonly curves around objects. That is why you
can hear a person speaking from the other side of a tree, or why you can hear around the corner of a building.
Only large objects – like mountains – will block and reflect sound waves.
Dispersion of light
The separation of white light
into its rainbow of colours. This property was first used by Sir Isaac Newton to show that white light is a mixture of colours.
Distortion of sound
Distortion occurs when the tones that were present in the original sound are not faithfully reproduced by the loudspeaker, or new tones are added that did not exist in
the original sound. Distortion often occurs because the material of the loudspeaker cone is not flexible enough, or because the coil in
the base of the loudspeaker is not sufficiently free to move.
Diverging lens
(See: Concave lens.)
Dome
A dome-shaped part of a loudspeaker. A dome can be used instead of a cone. It is found
in some smaller loudspeakers. A dome can be made from paper, fabric, thin aluminium or titanium.
Doppler effect of light
A shift in wavelength that occurs when light arrives from a source that is travelling very fast towards or away from the observer. The Doppler effect of light does not affect our everyday experience and is really only of concern to astronomers. They can use the Doppler effect to find out how
fast objects in space are travelling. A source moving away from the observer very fast is seen as a redder light than usual. That is called the red shift. It has been used as evidence for suggesting that the universe is expanding, because all stars show a red shift, so they must all be travelling away from us very fast.
Doppler effect of sound
A change in pitch caused by a sound source moving towards or away from an observer. The pitch is higher when the source and observer are getting closer and lower when they are moving apart. You hear the Doppler shift when
a fast-moving train or vehicle approaches and then passes.
For example, if a racing car is moving towards the listener, more sound waves enter the listener’s ear each second than the car engine produces. The sound seems higher pitched to the listener than it does to the driver.
When the racing car is moving away from the listener, fewer
sound waves enter the listener’s ear each second than the car engine produces. The sound now seems lower to the listener than it does to the driver. This explains the familiar ‘eeeeoooowwwwww’ sound as the car passes.
Dropout
A term used by hi-fi enthusiasts for a very weak response in part of
a loudspeaker or amplifier range. If the sound system has a very noticeable dropout, it can spoil listening pleasure.
Sound waves
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This part of the outer ear (called the pinna) funnels
sound waves onto
the eardrum. Outer ear