Thomas Young proved light is a wave using sunlight and a card
In November 1803 Thomas Young told the Royal Society his experiments needed nothing more than sunshine and everyday objects. By sliding a thin card into a beam from a window opening, he produced coloured fringes that vanished when either edge was blocked, a simple demonstration that challenged Newton's century-old view of light as particles.
Young, born in 1773 into a Quaker family in Milverton, Somerset, was the eldest of ten children. By 14 he had learned Greek, Latin, French, Italian, Arabic, Persian, Turkish, Syriac, Ge'ez and several other languages. He studied medicine in London, Edinburgh and Göttingen, earning his doctorate in 1796, and an inheritance from a great-uncle made him financially independent. He practised as a physician on Welbeck Street and published early scientific articles anonymously so his medical reputation would not suffer.
He judged his greatest achievement to be establishing the wave theory of light that Christiaan Huygens had set out in 1690. Ripple tanks showed interference in water; light experiments, forerunners of the famous double-slit setup, showed the same effect, and his measurements gave wavelengths for different colours close to modern figures. He also studied interference in soap films and Newton's rings. Within a decade Augustin-Jean Fresnel had reproduced and extended the work.
In 1807 he described the measure of stiffness now called Young's modulus, which depends only on the material rather than an object's shape, letting engineers predict how much a part would stretch under a known load without testing each new component. Giordano Riccati had used the concept experimentally in 1782 and Leonhard Euler anticipated it in 1727. Young's own definition was so convoluted that an Admiralty clerk wrote back that it was too learned to be understood; a contemporary likewise called him unusually poor at communicating what he knew.
His range was astonishing. As Royal Institution professor from 1801 he delivered 91 lectures in two years, and later he was secretary to the Board of Longitude, served on a committee weighing the risks of gas lighting in London, and advised on life insurance. He was instrumental in the early decipherment of Egyptian hieroglyphs on the Rosetta Stone. He died in London in 1829, and Westminster Abbey holds a marble tablet in his memory.
Source: Thomas Young (scientist)
