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Afterglow
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Aberration
A flaw in a lens. The lens does
not produce an accurate, clear image. For example, if the lens
is not the right shape, a straight line may appear curved when seen near the edge of the lens. If the line is curved towards the centre of the lens, the problem is called barrel distortion; if it is curved away from the centre, it is called pincushion distortion.
Absorption of light
The change of light striking an object into heat. As a result, the amount of light gradually lessens. If we shine a beam of light over a long distance, some of the light will be soaked up, or absorbed, by the water droplets and dust particles
in the air. As a result, the light
will appear fainter. It was partly
to get around this problem that
the Hubble telescope was sent into space. Hubble ‘sees’ many more faint objects than can be seen by telescopes on Earth because light from the objects is not absorbed by the atmosphere.
Some materials just absorb one kind of light. They are called filters. A red filter, for example, absorbs all lengths of light except red. Water absorbs red light, which
is why sea water appears blue.
The ice in an ice cave or a
crevasse appears blue for the same reason. (See also: Colour filters and Reflection.)
Acoustics
The scientific study of sound. The word acoustics comes from the Greek word akoustikos, meaning related to hearing.
The word acoustics is also used more narrowly to mean the effect
of objects or walls on music
or speech. If a room contains a large amount of sound-absorbing material, such as curtains and carpets, the reflections of the sound are lessened, and the acoustics are
 Absorption of light – Ice and water absorb red light, so the sea and glacier ice appear blue.
described as ‘dead’. If the room has hard, bare surfaces, such as in a church, sound is readily reflected and echoes become important. The acoustics of this type of room are called ‘live’. (See also: Anechoic chamber and Reverberation.)
Afterglow
The glow that appears in the western sky just after sunset, and which can last for up to an hour. Even when the Sun has set as
far as people on the ground are concerned, it has not set for people in a plane high above. Sunlight still reaches the upper sky long after the Sun has set on the ground. If the sunlight hits tiny particles
of dust and ice in the upper air, some of the light can be reflected down to the ground, producing a glow. (A glow also appears before sunrise in the eastern sky for the same reason.)
Afterglows are most dramatic when the air has a lot of dust in it, for example, when it has not rained for a long time, and especially soon after a volcanic eruption, when the volcano has shot lots of dust high into the air.
 Afterglow – An evening afterglow.
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