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Lighthouse
Light bulb
(See: Electric light.) Light energy
Light is a form of energy that can travel through empty space, or a transparent material, at very high speed. It is very different from
the chemical energy stored in coal or oil, for example, which can
only be transported by moving the material around. Light is, therefore, an extremely useful form of energy. It is available in huge amounts in sunshine – about 1.36 kilowatts per hour fall on every square metre at the equator, for example. The main problem is harnessing it because
it has to be converted into other forms of energy, such as heat or electricity, before we can use it. So far, methods of converting light into heat and electricity have had only limited success.
To convert light into heat, huge convex mirrors are used to focus the light on tubes of water. The water is heated to steam, and
then the steam is used to turn electricity generators. For this to be successful, a sunny site is needed.
To convert sunlight into electricity directly, materials called semi-conductors must be used.
Light-gathering power
The higher the light-gathering power of a lens or mirror, the easier it is to see small, faint objects. The light-gathering power of a telescope is made greater
by increasing the size of the
main lens or the main mirror. By doubling the diameter of a lens
or mirror, the light-gathering power is increased fourfold. (See also: Resolving power.)
 Lighthouse – A tower containing a light surrounded by a number of rotating Fresnel lenses.
Lighthouse
A structure, often a tower, that
acts as a beacon to warn ships of difficulties in navigation, such as shoals or the entrance to a harbour.
Most lighthouses use tungsten– halogen lamps, with power up to 1,500 watts. But a lamp on its own is not an efficient way of sending
a beam of light up to 40km out
to sea (the limit of visibility due to the curvature of the Earth). For this a system of mirrors and lenses is needed.
Concave mirrors are placed behind the lens. They collect light shining inland and convert it into a beam that is reflected out to sea.
The lamp is placed at the focus of the mirror.
The light beam can be focused further by a lens placed in front of it. In 1828 Augustin-Jean Fresnel made a lens panel consisting of
a central bull’s-eye lens with
a number of concentric glass prismatic rings (see: Prism). The panel concentrated the light into a pencil-like beam of great intensity. (See also: Fresnel lens.)
The problem with using a lens and a mirror to turn the light into a narrow beam is that it can only be seen from a small angle. To make the light visible from a wider angle, the whole lens, lamp and mirror system are rotated. As a result, the beam sweeps the horizon and is seen as a flash by people at sea.
By altering the shape of the lens further, flashes of different length can be obtained. In this way the flashes can be made into a code that can identify the lighthouse.
Lenses rotate
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