The furthest point from an eye at which an object can be seen in focus by the eye. The far point of a normal eye is at infinity.
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Filament bulb
Electrical device designed to produce light.
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Fluid
A liquid or gas.
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Focal length
The distance from the centre of a lens to the point where light rays parallel to the principal axis are focused (or in the case of a diverging lens, appear to diverge from).
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Force
A force can change the motion of an object (in newtons (N)).
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Fossil fuel
Fuel obtained from long-dead biological material.
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Free electron
Electron that moves about freely inside a metal and is not held inside an atom.
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Frequency
The number of wave crests passing a fixed point every second.
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Frequency (of an alternating current)
The number of complete cycles an alternating current passes through each second. The unit of frequency is the hertz (Hz).
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Friction
Force opposing the movement of one surface over another.
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Fuse
A fuse contains a thin wire that melts and cuts the current off if too much current passes through it.
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