Augustin-Jean Fresnel (10 May 1788 – 14 July 1827), was a French engineer who contributed significantly to the establishment of the theory of wave optics. Fresnel studied the behaviour of light both theoretically and experimentally.

He is perhaps best known as the inventor of the Fresnel lens, first adopted in lighthouses while he was a French commissioner of lighthouses, and found in many applications today.

Fresnel was the son of an architect, born at Broglie (Eure). His early progress in learning was slow, and he still could not read when he was eight years old. At thirteen he entered the École Centrale in Caen, and at sixteen and a half the École Polytechnique, where he acquitted himself with distinction. From there he went to the École des Ponts et Chaussées. He served as an engineer successively in the departments of Vendée, Drôme and Ille - et - Vilaine; but having supported the Bourbons in 1814 he lost his appointment on Napoleon's return to power.

In 1815 on the second restoration of the monarchy he obtained a post as engineer in Paris, where he spent much of his life from that time onwards. He appears to have begun his research in optics around 1814 when he prepared a paper on the aberration of light, although it was never published. In 1818 he wrote a memoir on diffraction for which he received the prize of the Académie des Sciences at Paris in the ensuing year. He was the first to construct a special type of lens, now called a Fresnel lens, as a substitute for mirrors in lighthouses. In 1819 he was nominated to be a commissioner of lighthouses. In 1823 he was unanimously elected a member of the academy, and in 1825 he became a member of the Royal Society of London. In 1827, the time of his last illness, the Royal Society of London awarded him the Rumford Medal.

Fresnel died of tuberculosis at Ville - d'Avray, near Paris.

He received only scant public recognition during his lifetime for his labours in the cause of optical science. Some of his papers were not printed by the Académie des Sciences until many years after his death. But as he wrote to Young in 1824: in himself "that sensibility, or that vanity, which people call love of glory" had been blunted. "All the compliments," he says, "that I have received from Arago, Laplace and Biot never gave me so much pleasure as the discovery of a theoretic truth, or the confirmation of a calculation by experiment". His name is one of the 72 names inscribed on the Eiffel Tower.

His discoveries and mathematical deductions, building on experimental work by Thomas Young, extended the wave theory of light to a large class of optical phenomena. In 1817, Young had proposed a small transverse component to light, while yet retaining a far larger longitudinal component. Fresnel, by the year 1821, was able to show via mathematical methods that polarization could be explained only if light was entirely transverse, with no longitudinal vibration whatsoever.

He proposed the aether drag hypothesis to explain a lack of variation in astronomical observations.

His use of two plane mirrors of metal, forming with each other an angle of nearly 180°, allowed him to avoid the diffraction effects caused (by the apertures) in the experiment of F.M. Grimaldi on interference. This allowed him to conclusively account for the phenomenon of interference in accordance with the wave theory.

With François Arago he studied the laws of the interference of polarized rays. He obtained circularly polarized light by means of a rhombus of glass, known as a Fresnel rhomb, having obtuse angles of 126° and acute angles of 54°.