Young, Thomas (1773–1829)

English physicist, physician, and Egyptologist who revived the wave theory of light and identified the phenomenon of interference in 1801. He also established many important concepts in mechanics.

In 1793, Young recognized that focusing of the eye (accommodation) is achieved by a change of shape in the lens of the eye, the lens being composed of muscle fibres. He also showed that astigmatism is due to irregular curvature of the cornea. In 1801, he became the first to recognize that colour sensation is due to the presence in the retina of structures that respond to the three colours red, green, and violet.

Young was born in Milverton, Somerset. A child prodigy, he had learned most European and many ancient languages by the age of 20. He studied medicine in London and at Edinburgh and Göttingen, Germany. He was professor of natural philosophy at the Royal Institution 1801–03 and worked as a physician at St George's Hospital, London, from 1811. Young assumed that light waves are propagated in a similar way to sound waves, and proposed that different colours consist of different frequencies. He obtained experimental proof for the principle of interference by passing light through extremely narrow openings and observing the interference patterns produced. In mechanics, Young was the first to use the terms ‘energy’ for the product of the mass of a body with the square of its velocity and ‘labour expended’ for the product of the force exerted on a body ‘with the distance through which it moved’. He also stated that these two products are proportional to each other. He introduced an absolute measurement in elasticity, now known as Young's modulus. From 1815 onwards, Young published papers on Egyptology; his account of the Rosetta stone played a crucial role in the stone's eventual decipherment.