A burning glass or burning lens is a large convex lens that can concentrate the sun's rays onto a small area, heating up the area and thus resulting in ignition 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.
Burning glass technology has been known since antiquity, as described by Greek and Roman writers who recorded the use of lenses to start fires for various purposes. Pliny the Elder noted the use of glass vases filled with water to create a heat intense enough to ignite clothing, as well as convex lenses that were used to cauterize wounds.  Plutarch refers to a burning mirror made of joined triangular metal mirrors installed at the temple of the Vestal Virgins. Aristophanes mentions the burning lens in his play The Clouds (424 BC).
- "Strepsiades. Have you ever seen a beautiful, transparent stone at the druggists', with which you may kindle fire?"
Archimedes, the renowned mathematician, was said to have used a burning glass as a weapon in 212 BC, when Syracuse was besieged by Marcus Claudius Marcellus. The Roman fleet was supposedly incinerated, though eventually the city was taken and Archimedes was slain.
The legend of Archimedes gave rise to a considerable amount of research on burning glasses and lenses until the late 17th century. Various researchers 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), Alhazen in his Book of Optics (1021), 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.
Burning lenses were used both by 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.
In 1796, during the French Revolution and three years after the declaration of war between France and Great Britain, 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.
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 pop science TV program MythBusters attempted to model Archimedes' feat by using mirrors to ignite a small wooden boat covered with tar, with little success—they found it too difficult to focus light from their hand-held mirrors onto a point small enough to ignite the boat. 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, though not made of the same quality of materials as in the MythBusters effort.
Burning glasses (often called fire lenses) are still used to light fires in outdoor and primitive settings. Large burning lenses sometimes take the form of Fresnel lenses, similar to lighthouse lenses, including those for use in solar furnaces. Solar furnaces are used in industry to produce extremely high temperatures without the need for fuel or large supplies of electricity. They sometimes employ a large parabolic array of mirrors (some facilities are several stories high) to focus light to a high intensity.
- Sherwood, Andrew N.; Nikolic, Milorad; Humphrey, John W.; Oleson, John P. (2019). Greek and Roman Technology: A Sourcebook of Translated Greek and Roman Texts. Routledge. p. 25. ISBN 978-1138927896.
- Plantzos, Dimitris (1997). "Crystals and Lenses in the Graeco-Roman World". American Journal of Archaeology. 101 (3): 451–464. doi:10.2307/507106.
- Aristophanes. "The Clouds". The Internet Classics Archive. Daniel C. Stevenson, Web Atomics. Retrieved 21 February 2018.
- Meijer, Fik (1986), A History of Seafaring in the Classical World, Routledge, ISBN 978-0-7099-3565-0
- Hone, William (1857). The table book: or, Daily recreation and information concerning remarkable men, manners, times, seasons, solemnities, merry-makings, antiquities and novelties, forming a complete history of the year. London: William Tegg and Co. p. 813.
- Roshdi Rashed (1990), "A Pioneer in Anaclastics: Ibn Sahl on Burning Mirrors and Lenses", Isis 81 (3), p. 464-491 [464-468].
- Joseph Priestley, Experiments and Observations on Different Kinds of Air Vol.2 (1776)
- Burns, Paul. "The History of The Discovery of Cinematography: Chapter Six 1750-1799". Accessed 29 July 2007.
- Bates, William H. (1920). "Chapter 17: Vision Under Adverse Conditions a Benefit to the Eye". Perfect Sight Without Glasses. New York: Central Fixation Publish Co. pp. 183–197. ISBN 88-900756-3-5.
- "Fire Glass". Primitivefire.com. Retrieved 2012-12-18.
- George A. Macbeth (1914). "Light House Lenses". Proceedings of the Engineers' Society of Western Pennsylvania. 30: 245.
- H. P. Garg, J. Prakash (2000). Solar Energy: Fundamentals and Applications. Tata McGraw-Hill Education. p. 305. ISBN 9780074636312.
- "Olympic Torch Relay history". London 2012 Olympic Games. Retrieved 27 November 2012.