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How do lighthouses work?

Despite the continual development of technologies – radio beacons, radar, GPS – it is still lighthouses that illuminate our coastline and are the last resort for sailors when their sophisticated equipment is faulty. Let’s shine a spotlight on these structures which have dotted the French coast since the end of the 18th century.



                           escalier de Phare
                        
                           Photo by Chantal de Bruijne / Shutterstock.com



The history of lighthouses most probably began in Antiquity, in the Mediterranean. At first they were simple wood fires set alight on clifftops in the open air, then later atop towers specially built for the purpose, such as the famous lighthouse of Alexandria on the isle of Pharos. Lighthouses evolved along with their means of lighting: charcoal replaced wood, the oil lamp replaced charcoal, and electricity replaced the oil lamp. But even though these light sources became more and more intense and increasingly less difficult to maintain, a technology was needed which was able to increase the range of the light beam emitted. To this end, an oil lamp was placed at the centre of a parabolic reflector made of metal. This technique only increased the beam’s range a little because the metal used to reflect it also absorbed a large amount of the energy emitted by the lamp. Then came a Frenchman with a bright idea which would see the performance of lighthouses improve significantly – in fact they would shine 4.5 million times more intensely.



Créac'h Lighthouse
Photo by Maurizio Biso / Shutterstock.com

Créac'h Lighthouse, on île d'Ouessant, the most powerful lighthouse in the world.


In 1821, Augustin Fresnel, an engineer for the body who commissioned lighthouses, suggested replacing the metallic reflectors by stepped lenses. These lenses, now known as Fresnel lenses, are made up of a central convex disc (curving outwards) encircled by a series of concentric rings which act as prisms, focusing the rays of a light source onto a single plane. As they leave the lens, the light rays refracted by the concentric rings are parallel to each other and travel in the same direction. This process, therefore, allows illumination of just the horizon without diffusion of light in other directions, and thus the intensity of the beams is increased by up to a power of 4.5 million! It proved so successful that it was installed progressively in lighthouses across the world and still remains to this day the origin of the many beams of light that sweep our skies.



Fresnel Lens
Photo by Stephane Bidouze / Shutterstock.com

Créac'h Lighthouse's Fresnel lens




Créac'h Lighthouse
Photo by Stephane Bidouze / Shutterstock.com

Créac'h Lighthouse, on île d'Ouessant, the most powerful lighthouse in the world.



A navigational language

Lighthouses signal the presence of rocks or dangerous coastal zones. But lighthouses do not just light the coastline; they also allow sailors to get their bearings. Each lighthouse possesses its own characteristics, its own way of lighting the horizon. Some are sector lights, which emit different colours from different angles: red or green to indicate danger zones, white to indicate the direction to follow. Those which only emit a white light can be: fixed lights (with a constant light intensity in every direction); flashing lights (periods of darkness longer than periods of light); isophase lights (identical durations of light and darkness), or occulting lights (periods of light longer than periods of darkness). So sailors, in order to work out their position, have only to determine the type of light, count the length of the periods of darkness and light, and measure the rhythm of these periods. By referring to a book in which all the lighthouse characteristics are listed, they will be able to deduce the name of the lighthouse they can see and in turn their location in relation to the coast. Useful if their GPS or radar are not functioning.

Light beam travelling through a Fresnel lens

Sophie Fromager,

Centre de vulgarisation de la connaissance (Université Paris-Saclay)
Service COmmunication, Médiation et PAtrimoine Scientifiques (COMPAS)
Faculté des sciences
Bâtiment 301 
Université Paris-Sud
91 405 Orsay Cedex
Tél : 0033 1 69 15 66 19

www.cvc.u-psud.fr

Article from lEspace des sciences 
Les Champs Libres
10 cours des Alliés
35 000 Rennes
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www.espace-sciences.org

Espace des Sciences Rennes