Thomas Young established the principle of interference of light and thus resurrected the century-old wave theory of light. He was also an Egyptologist who helped decipher the Rosetta Stone.

From arts to medicine
Thomas Young's father was a banker and Young was brought up as a Quaker. He was a precocious child, learning to read by the age of two. He attended two boarding schools between 1780 and 1786 where his ability to learn languages became marked. He early also possessed extensive knowledge of mathematics, and natural sciences. For the next few years he studied privately; his reading included the works of Isaac Newton (1642-1727), Antoine-Laurent Lavoisier (1743-1794) and Joseph Black (1728-1799). In 1793, on the advice of his nephew Dr. Richard Brocklesby, he entered St Bartholomew's Hospital, London, to study medicine. As a medical student he attended the lectures of John Hunter (1728-1793), William Cruikshank (1745-1800), Matthew Baillie (1761-1823), and others. He continued this study at Edinburgh from 1794, and from 1795 in Göttingen. It is from this period that he began to distance himself from Quakerism. He obtained his medical doctorate in 1796.

From 1797 to 1803 Young was attached to Emmanuel College Cambridge (MR 1803, MD 1808), where he turned his attention to scientific matters. In 1797 an uncle left him 10,000 pounds and a London house into which he moved in 1800 and in 1804 he married Eliza Maxwell.

In 1799 Young set up a medical practice in London. His primary interest was in sense perception, and, while still a medical student, he had discovered the way in which the lens of the eye changes shape to focus on objects at differing distances. In 1801 he showed that astigmatism results from an improperly curved cornea. The same year he turned to the study of light.

As early as around 1790, prior to the discovery of the cone cells in the retina, Young introduced the original theory of colour. What is now referred to as the Young-Helmholtz theory, was first published in 1802 by Thomas Young. It is based on the assumption that there are three fundamental colour sensations—red, green, and blue—and that there are three different groups of cones in the retina, each group particularly sensitive to one of these three colours. Light from a red object, for example, stimulates the cones that are more sensitive to red than the other cones. Other colours (besides red, green, and blue) are seen when the cone cells are stimulated in different combinations. Only in recent years has conclusive evidence shown that the Young-Helmholtz theory is, indeed, accurate. The sensation of white is produced by the combination of the three primary colours, and black results from the absence of stimulation. The theory was later further developed by Hermann von Helmholtz.

In 1793 his paper On vision was read in the Royal Society and printed in its Transactions the

 same year. This occasioned his election to fellow of the same society in 1794, at the age of only 21 years. He received his doctorate at Göttingen in 1796, was accepted to the College at Cambridge, and was appointed professor of physics at the Royal Institution.

By 1801, aged 28, Thomas Young was already professor of natural philosophy at the Royal Institution and lecturing on acoustics, optics, gravitation, astronomy, tides, the nature of heat, electricity, climate, animal life, vegetation, cohesion and capillary attraction of liquid, the hydrodynamics of reservoirs, canals and harbours, techniques of measurement, common forms of air and water pumps, and new ideas on energy. Had enough?

In 1802 (1804?) Young became foreign secretary of the Royal Society, holding this position for the rest of his life. He was conferred doctor of medicine at Cambridge in 1808, and in 1809 was elected fellow of the College of Physicians, at which he was twice censor and twice (1822, 1823) Croonian lecturer. From 1811 until his death he was physician at the St. George’s Hospital, however, without contributing anything of importance as a clinician. His medical works of 1813 and 1823 are mere compilations without any investigations of his own.

From 1818 he was secretary of the Board of Longitude and entrusted the supervision of the nautical almanac published by the admiralty, a position he retained following the abolition of the Board in 1828. After the abolition he was named one of the Admiralty's scientific advisers.

The physician who saw the light
It was a short step from physiological optics to considering the nature of light. Young's interest in this was reinforced by some work he had done in the mid-1790s on the transmission of sound which he came to believe was analogous to light. In 1802 Young first demonstrated a simple proof of the wave theory of light. He forced the light from a single light source to pass through a narrow slit and then forced that same light to pass through two more narrow slits placed within a fraction of an inch of each other. The light from the two slits fell on a screen. Young found that the light beams spread apart and overlapped, and, in the area of overlap, bands of bright light alternated with bands of darkness.

With this demonstration of the interference of light, Young definitely established the wave nature of light. He used his new wave theory to explain the colours of thin films (such as soap bubbles), and, relating colour to wavelength, he calculated the approximate wavelengths of the seven colours recognized by Newton. In 1817 he proposed that light waves were transverse (vibrating at right angles to the direction of travel), rather than longitudinal (vibrating in the direction of travel) as had long been assumed, and thus explained polarization, the alignment of light waves to vibrate in the same plane.

With his discovery, however, Young came into conflict with the theories of Sir Isaac Newton, who tried to explain optical phenomena such as refraction and reflection in terms of gravitational-like effects. As it turned out later, in a way, Newton's theory was given partial confirmation by the Quantum Theory. In the early 19th century, however, any opposition to a theory of Newton's was unthinkable by most English scientists. Ridiculed in England, Young’s theory was championed in France by Augustin Jean Fresnel (1788-1827) and Dominique-François-Jean Arago (1785-1853), and finally achieved acceptance in Europe. A savage anonymous review of his work in 1803 in the Edinburgh Review (now known to have been due to Lord Henry Peter Brougham (1778-1868), a proponent of the corpuscular theory) cast Young into scientific limbo for ten years.

The Egyptologist
After his work on optics, Young returned to the study of languages, becoming particularly interested in Egyptology. From 1813 (1814?) he started to attempt to decipher Egyptian hieroglyphics. Young began studying the texts of the Rosetta Stone in 1814. After obtaining additional hieroglyphic writings from other sources, he succeeded in providing a nearly accurate translation within a few years and thus contributed heavily to deciphering the ancient Egyptian language.

However, he published little at the time due, it would seem, to his official duties. In 1823 he published a comparison of his and J.F. Champollion's work and his Enchorial Egyptian Dictionary was published posthumously in 1830. Both were attempts to claim priority over Champollion and, in so far as Young is generally mentioned when the deciphering of hieroglyphics is discussed, he was not wholly unsuccessful.

Young also did work on measuring the size of molecules, surface tension in liquids, and on elasticity. He was the first to give the word energy its scientific significance, and Young's modulus, a constant in the mathematical equation describing elasticity, was named in his honour.

Thomas Young, as his epitaph in Westminster Abbey states, was "a man alike eminent in almost every department of human learning."

Leisure and application are the great requisites for improving the mind: leisure is useless without application; but application with a very little leisure may produce very material benefit. If you are careful of your vacant minutes, you may advance yourselves more than many do who have every convenience afforded them.
Quoted in Biographical Memoirs of the Most Celebrated Physicians and Surgeons, Volume IV.

Associated eponyms:
Young's modulus
A constant in the mathematical equation describing elasticity.

Young's rule
A “rule of the thumb” method for calculating the dose of medicine to be administered to a child.

Young's temperament (Thomas Young)
A keyboard tuning.

Young-von Helmholtz three colour theory
A theory of colour perception presented by Thomas Young in 1802 and later established psycho-physiologically by von Helmholtz.

Bibliography:

• Observations on vision.
  Philosophical Transactions, 1793, 83: 169-181.
• De corporis humani viribus conservatricibus.
  Doctoral dissertation, Göttingen, 1796.
• On the mechanism of the eye.
  Philosophical Transactions, 1801, 91: 23-28.
  Includes the first description of astigmatism, with measurements and optical constants
• On the theory of light and colours. Philosophical Transactions, 1802, 92: 12-48.
• A Course of Lectures on Natural Philosophy and the Mechanical Arts.
  2 volumes, 4, London, 1807; 1845 published by P. Kelland.
  The most complete and exact work on physics in his time.
• An introduction to medical literature, including a system of practical nosology. Intended as a guide to students, and an assistant to practitioners.
  London, 1813; 2nd edition, 1823.
  Treatise on consumptive diseases. 1815.
• Herbert Spencer Robinson:
  Thomas Young. A Chronology and Bibliography with Estimates of his Work and Character.
  Med. Life, 1929; 36: 526-540.