Young , Thomas

(1773–1829) British physicist, physician, and Egyptologist

Young, who was born at Milverton in southwest England, was a child prodigy. He could read with considerable fluency at the age of 2 and by 13 he had a good knowledge of Latin, Greek, French, and Italian. He had also begun to study natural history and natural philosophy and could make various optical instruments. A year later he began an independent study of the Hebrew, Chaldean, Syriac, Samaritan, Arabic, Persian, Turkish, and Ethiopic languages. When he was 19 he was a highly proficient Latin and Greek scholar, having mastered many literary and scientific works including Newton's Principia and Opticks and Antoine Lavoisier's Traité élémentaire de chimie (Elementary Treatise on Chemistry).

In 1793 Young began a medical education, studying first at St. Bartholomew's Hospital, London, and then at the universities of Edinburgh (1794), Göttingen (1795), and Cambridge (1797). In 1800, after receiving a considerable inheritance, he set up a medical practice in London; this practice, however, never really flourished. In 1801 he was appointed professor of natural philosophy at the Royal Institution. Although his lectures were erudite, and remarkable for their scope and originality, they were not successful. In 1803 Young resigned his post. From then until his death he held various medical appointments and several offices related to science.

Young's early scientific researches were concerned with the physiology of the eye. He was elected a fellow of the Royal Society in 1794 for his explanation of how the ciliary muscles change the shape of the lens to focus on objects at differing distances (accommodation); in 1801 he gave the first descriptions of the defect astigmatism and of color sensation.

Young's most lasting contribution to science was his work in helping to establish the wave theory of light. Between 1800 and 1804 he revived an interest in this theory and gave it strong support. He compared the ideas of Newton and Christiaan Huygens on the nature of light, criticizing the corpuscular theory for its inadequacy in explaining such phenomena as simultaneous reflection and refraction. He introduced the idea of interference of light, which he explained by the superposition of waves – a principle that he applied to a range of optical phenomena including Newton's rings, diffraction patterns, and the color of the supernumerary bows of the rainbow. In his best-known demonstration of interference he passed light first through a single pinhole, then through two further pinholes close together; the light then fell upon a screen and gave a series of light and dark bands. The apparatus is known as Young's slits. Young's views were very badly received in England, where opposition to Newton's corpuscular theory was unthinkable.

From about 1804 Young devoted himself more to medical practice and the study of philology, especially the decipherment of hieroglyphic writing. He made very important contributions to the latter field and, independently of Jean François Champollion, helped in translating the text of the Rosetta Stone. His interest in optics was revived in about 1816 by the work of François Arago and Augustin Fresnel. In a letter to Arago he suggested that light might be propagated as a transverse wave (in which the vibrations of the medium are perpendicular to the direction of propagation). This allowed polarization to be explained on the wave theory and gave a satisfactory explanation of the known optical phenomena. The decisive test on the nature of light came later when its speed in the air and water could be accurately measured. Young's other scientific contributions included researches into sound, capillarity, and the cohesion of fluids. Because he gave a physical meaning to the constant of proportionality (E) in Hooke's law, E is called Young's modulus.