Young, Thomas, 1773–1829, English physicist, physician, and Egyptologist. He established (1799) a medical practice in London and was elected (1811) to the staff of St. George's Hospital there. His lectures while professor of natural philosophy (1801–3) at the Royal Institution, London, published as A Course of Lectures on Natural Philosophy and the Mechanical Arts (1807), introduced the modern physical concept of energy. An authority on the mechanism of vision and on optics, he stated (1807) a theory of color vision now known as the Young-Helmholtz theory, studied the structure of the eye, and described the defect called astigmatism. He is especially noted for reviving the wave theory of light as opposed to the corpuscular theory, advancing as proof a demonstration based upon the principle of interference of light, which he first formulated in 1801. He applied (1809) the wave theory to refraction and dispersion phenomena. Young's versatility is evidenced by his contributions to the theory of tides, his participation in the deciphering of the Rosetta Stone (see under Rosetta), which provided a key to understanding Egyptian hieroglyphic writings, his explanation (1804) of capillarity (independently set forth by Laplace in 1805), and his establishment of a coefficient of elasticity, Young's modulus.
See biographies by H. B. Williams (1930) and A. Wood (1954).