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A burning glass or burning lens is a large Biconvex lens that can concentrate the Sun's rays onto a small area, heating up the area and thus resulting in Combustion of the exposed surface. Burning mirrors achieve a similar effect by using reflecting surfaces to focus the light. They were used in 18th-century chemical studies for burning materials in closed glass vessels where the products of combustion could be trapped for analysis. The burning glass was a useful contrivance in the days before electrical ignition was easily achieved.
The Hellenistic Greek mathematician Archimedes was said to have used a burning glass as a weapon in 212 BC, when Syracuse was besieged by Marcus Claudius Marcellus of the Roman Republic. The Roman Navy was supposedly incinerated, though eventually the city was taken and Archimedes was slain.Meijer, Fik (1986), A History of Seafaring in the Classical World, Routledge,
The legend of Archimedes gave rise to a considerable amount of research on burning glasses and lenses until the late 17th century. Various researchers from medieval Christendom to the Islamic world worked with burning glasses, including Anthemius of Tralles (6th century AD), Proclus (6th century; who by this means purportedly destroyed the fleet of Vitalian besieging Constantinople), Ibn Sahl in his On Burning Mirrors and Lenses (10th century), Ibn al-Haytham in his Book of Optics (1021),Roshdi Rashed (1990), "A Pioneer in Anaclastics: Ibn Sahl on Burning Mirrors and Lenses", Isis 81 (3), pp. 464–491 464–468. Roger Bacon (13th century), Giambattista della Porta and his friends (16th century), Athanasius Kircher and Gaspar Schott (17th century), and the Comte de Buffon in 1740 in Paris.
While the effects of camera obscura were mentioned by Greek philosopher Aristotle in the 4th century BC, contemporary Chinese of China's Warring States Period who compiled the Mozi described their experiments with burning mirrors and the pinhole camera. A few decades after Alhazen described camera obscura in Iraq, the Song dynasty Chinese statesman Shen Kuo was nevertheless the first to clearly describe the relationship of the focal point of a concave mirror, the burning point and the pinhole camera as separate radiation phenomena in his Dream Pool Essays (1088). By the late 15th century Leonardo da Vinci would be the first in Europe to make similar observations about the focal point and pinhole.
Burning lenses were used in the 18th century by both Joseph Priestley and Antoine Lavoisier in their experiments to obtain oxides contained in closed vessels under high temperatures. These included carbon dioxide by burning diamond, and mercuric oxide by heating mercury. This type of experiment contributed to the discovery of "dephlogisticated air" by Priestley, which became better known as oxygen, following Lavoisier's investigations.Joseph Priestley, Experiments and Observations on Different Kinds of Air Vol.2 (1776)
Chapter 17 of William Bates' 1920 book Perfect Sight Without Glasses, in which the author argues that observation of the sun is beneficial to those with poor vision, includes a figure of somebody "Focussing the Rays of the Sun Upon the Eye of a Patient by Means of a Burning Glass."
The burning lens of the Grand Duke of Tuscany was used by Sir Humphry Davy and Michael Faraday to burn a diamond in oxygen on 27 March 1814.
In 1796, during the French Revolution and three years after the declaration of war between France and Great Britain, physicist Étienne-Gaspard Robert met with the French government and proposed the use of mirrors to burn the invading ships of the British Royal Navy. They decided not to take up his proposal.Burns, Paul. " The History of The Discovery of Cinematography: Chapter Six, 1750–1799". Accessed 29 July 2007.
From the 7th to the 16th centuries, a burning glass was used by Christians to set off the Easter Fire during the Easter vigil. Thus, Saint Boniface explained to Pope Zachary that he produced the new fire of Holy Saturday by means of a crystal lens concentrating the rays of the sun. This process was also mentioned in liturgical books until the Roman Pontifical of 1561.
In Cambodia, a burning glass has also been used since ancient times for the cremation of kings and most recently for the funeral of King Sihanouk. The crematorium of the king is traditionally prepared by the Bakus brahmin from the Royal Palace on the last day of the week-long funeral. Small pieces of fragrant agarwood are placed beneath the magnifying glass until it ignites. The incandescent wood is used to light candles and pass on the fire to the attendees, who usually take their lit candles home.
However, accounting for battle conditions makes such a weapon impractical, with modern tests refuting such claims. An experiment was carried out by a group at the Massachusetts Institute of Technology in 2005. It concluded that although the theory was sound for stationary objects, the mirrors would not likely have been able to concentrate sufficient solar energy to set a ship on fire under battle conditions. Similar experiments were conducted on the popular science-based TV show MythBusters in 2004, 2006, and 2010, arriving at similar results based on the premise of the controversial myth.
However, an episode of Richard Hammond's Engineering Connections relating to the Keck Observatory (whose reflector glass is based on the Archimedes' Mirror) did successfully use a much smaller curved mirror to burn a wooden model, although the scaled-down model was not made of the same quality of materials as in the MythBusters effort.
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