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WonderOptical engineering fact

A Fresnel lens replaces the thick curved glass of an ordinary lens with thin concentric rings, each shaped to bend light at the right angle. Invented for lighthouses in the 1820s, it focuses light just as strongly with far less glass and weight.

Cordouan Lighthouse, France, 1823

EngineeringTechnologyOceanDesign

The story behind the card

In 1819 the French physicist Augustin-Jean Fresnel, working for France's lighthouse commission, proposed a stepped lens to replace the metal reflectors then in use, which returned only about half of a lamp's light. A full-size lens was tested in Paris in 1822, and on 25 July 1823 the first lighthouse Fresnel lens was lit at Cordouan, at the mouth of the Gironde; its light could be seen more than 32 kilometres out to sea. A solid lens of the same power would have been impossibly thick and heavy. Later improved with glass prisms that also sent stray light towards the horizon, the design spread to lighthouses around the world, and it has been called the invention that saved a million ships.

Türkçe oku
Fresnel merceği, sıradan bir merceğin kalın ve kavisli camı yerine iç içe ince halkalar kullanır; her halka ışığı tam gereken açıyla kırar. 1820'lerde deniz fenerleri için geliştirilen bu mercek, ışığı çok daha az cam ve ağırlıkla aynı güçte toplar.
Cordouan Deniz Feneri, Fransa, 1823

Fransız fizikçi Augustin-Jean Fresnel, 1819'da Fransa'nın deniz fenerleri komisyonu için çalışırken o dönemde kullanılan metal yansıtıcıların yerine basamaklı bir mercek önerdi; bu yansıtıcılar lamba ışığının ancak yarısı kadarını geri yollayabiliyordu. Tam boyutlu bir mercek 1822'de Paris'te denendi. 25 Temmuz 1823'te ilk Fresnel merceği, Gironde Nehri'nin ağzındaki Cordouan Deniz Feneri'nde yakıldı; ışığı denizde 32 kilometreden daha uzaktan görülebiliyordu. Aynı güçte tek parça bir mercek akıl almaz kalınlıkta ve ağırlıkta olurdu. Sonradan başıboş ışığı da ufka yönelten cam prizmalarla geliştirilen tasarım bütün dünyanın deniz fenerlerine yayıldı ve “bir milyon gemiyi kurtaran buluş” diye anıldı.

Sources and further reading

Background reading: Fresnel lens
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