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Aperture
The amount of light allowed into a camera (see: f-number).
Aurora
A curtain of yellowish-green, white and red lights that appear to dance in the night sky close to the poles. The display is called the aurora borealis in the Northern Hemisphere and
the aurora australis in the Southern Hemisphere. It can be seen from Alaska and from places of similar latitude in North America and from northern Scandinavia and Russia. Occasionally it extends further south, when people as far south as northern Scotland can
see the effects. In the Southern Hemisphere the effect can only be seen in Antarctica.
Each part of the curtain is
a coloured band, several hundred kilometres long and stretching in the same direction as the Earth’s magnetic field. The bands keep appearing and disappearing within seconds.
The auroras occur as the solar wind (a flow of particles from the Sun outwards into space) is influenced by the Earth’s magnetic field.
B
Bass
The lowest male voice (see:
Human voice). The lowest-pitched member of the violin family is called a double bass. (See also: Alto; Tenor; Treble.)
Beam of light
Light rays that have been made to run parallel. That is
normally done by using reflectors and lenses.
The way a torch bulb focuses light into a beam is one example.
 Aurora – Coloured patterns produced as the Sun’s solar wind blows through the Earth’s magnetic field.
Beat
In music the basic rhythmic unit of a bar (for example, four beats to the bar).
In sound and light the shape
of waves produced by combining two waves (see: Light waves and Sound waves). It can be heard using a piano by striking a white key and its neighbouring black key
near the bass end of the keyboard. The combined notes appear to
fade out and then get stronger in a kind of throbbing pattern. They are the beats.
Beats are used to tune musical instruments. When a tuning fork and a piano string are hit at the same time, but the piano string is slightly out of tune, beats will be heard. When the string is exactly in tune, no beats are heard. They then have identical pitch.
 Beam of light – A lens or a concave mirror can be used to direct light from a source into a beam of parallel rays.
Beat
Beam of light Lens
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