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 concentrate sunlight heat intensely 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?

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 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 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), 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.

While the effects of camera obscura were mentioned by Greek philosopher Aristotle in the 4th century BC, contemporary Chinese Mohists 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.

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."