Thomas Young (13 June 1773 – 10 May 1829) was an English polymath and physician. Young made notable scientific contributions to the fields of vision, light, solid mechanics, energy, physiology, language, musical harmony, and Egyptology.


Wave theory of lightEdit

In Young's own judgment, of his many achievements the most important was to establish the wave theory of light.[1][2] To do so, he had to overcome the century-old view, expressed in the venerable Isaac Newton's Opticks, that light is a particle. Nevertheless, in the early-19th century Young put forth a number of theoretical reasons supporting the wave theory of light, and he developed two enduring demonstrations to support this viewpoint. With the ripple tank he demonstrated the idea of interference in the context of water waves. With the Young's interference experiment, or double-slit experiment, he demonstrated interference in the context of light as a wave.

"The experiments I am about to relate ... may be repeated with great ease, whenever the sun shines, and without any other apparatus than is at hand to every one", is how Thomas Young, speaking on 24 November 1803, to the Royal Society of London, began his description of the historic experiment. His talk was published in the following year's Philosophical Transactions,[3] and was destined to become a classic, still reprinted and read today.[4]

In the subsequent paper entitled Experiments and Calculations Relative to Physical Optics, published in 1804, Young describes an experiment in which he placed a narrow card (approximately 1/30th inch) in a beam of light from a single opening in a window and observed the fringes of colour in the shadow and to the sides of the card. He observed that placing another card before or after the narrow strip so as to prevent light from the beam from striking one of its edges caused the fringes to disappear.[5] This supported the contention that light is composed of waves.[6] Young performed and analysed a number of experiments, including interference of light from reflection off nearby pairs of micrometre grooves, from reflection off thin films of soap and oil, and from Newton's rings. He also performed two important diffraction experiments using fibres and long narrow strips. In his Course of Lectures on Natural Philosophy and the Mechanical Arts (1807) he gives Grimaldi credit for first observing the fringes in the shadow of an object placed in a beam of light. Within ten years, much of Young's work was reproduced and then extended by Fresnel. (Tony Rothman in Everything's Relative and Other Fables from Science and Technology argues that there is no clear evidence that Young actually did the two-slit experiment. See also Newton wave–particle duality.)

Young's modulusEdit

Young described the characterization of elasticity that came to be known as Young's modulus, denoted as E, in 1807, and further described it in his Course of Lectures on Natural Philosophy and the Mechanical Arts.[7]

However, the first use of the concept of Young's modulus in experiments was by Giordano Riccati in 1782 – predating Young by 25 years.[8]

Furthermore, the idea can be traced to a paper by Leonhard Euler published in 1727, some 80 years before Thomas Young's 1807 paper.

The Young's modulus relates the stress (pressure) in a body to its associated strain (change in length as a ratio of the original length); that is, stress = E × strain, for a uniaxially loaded specimen. Young's modulus is independent of the component under investigation; that is, it is an inherent material property (the term modulus refers to an inherent material property). Young's Modulus allowed, for the first time, prediction of the strain in a component subject to a known stress (and vice versa). Prior to Young's contribution, engineers were required to apply Hooke's F = kx relationship to identify the deformation (x) of a body subject to a known load (F), where the constant (k) is a function of both the geometry and material under consideration. Finding k required physical testing for any new component, as the F = kx relationship is a function of both geometry and material. Young's Modulus depends only on the material, not its geometry, thus allowing a revolution in engineering strategies.

Young's problems in sometimes not expressing himself clearly were shown by his own definition of the modulus: "The modulus of the elasticity of any substance is a column of the same substance, capable of producing a pressure on its base which is to the weight causing a certain degree of compression as the length of the substance is to the diminution of its length." When this explanation was put to the Lords of the Admiralty, their clerk wrote to Young saying "Though science is much respected by their Lordships and your paper is much esteemed, it is too learned ... in short it is not understood."[9]

Vision and colour theoryEdit

Young has also been called the founder of physiological optics. In 1793 he explained the mode in which the eye accommodates itself to vision at different distances as depending on change of the curvature of the crystalline lens; in 1801 he was the first to describe astigmatism;[10] and in his lectures he presented the hypothesis, afterwards developed by Hermann von Helmholtz, that colour perception depends on the presence in the retina of three kinds of nerve fibres.[11] This foreshadowed the modern understanding of colour vision, in particular the finding that the eye does indeed have three colour receptors which are sensitive to different wavelength ranges.

Young–Laplace equationEdit

In 1804, Young developed the theory of capillary phenomena on the principle of surface tension.[12] He also observed the constancy of the angle of contact of a liquid surface with a solid, and showed how from these two principles to deduce the phenomena of capillary action.

In 1805, Pierre-Simon Laplace, the French philosopher, discovered the significance of meniscus radii with respect to capillary action.

In 1830, Carl Friedrich Gauss, the German mathematician, unified the work of these two scientists to derive the Young–Laplace equation, the formula that describes the capillary pressure difference sustained across the interface between two static fluids.

Young was the first to define the term "energy" in the modern sense.[13]

Young's equation and Young–Dupré equationEdit

Template:Details Young's equation describes the contact angle of a liquid drop on a plane solid surface as a function of the surface free energy, the interfacial free energy and the surface tension of the liquid. Young's equation was developed further some 60 years later by Dupré to account for thermodynamic effects, and this is known as the Young–Dupré equation.


  1. "Thomas Young (1773–1829)". UC Santa Barbara. Retrieved 5 September 2016. 
  2. Haidar, Riad. "Thomas Young and the wave theory of light". Bibnum. Retrieved 5 September 2016. 
  3. Young, Thomas (1804). "Bakerian Lecture: Experiments and calculations relative to physical optics". Philosophical Transactions of the Royal Society 94: 1–16. doi:10.1098/rstl.1804.0001. Bibcode1804RSPT...94....1Y. 
  4. Shamos, Morris (1959). Great Experiments in Physics. New York: Holt Rinehart and Winston. pp. 96–101. 
  5. Magie, William Francis (1935). A Source Book in Physics. Harvard University Press.  p. 309
  6. Of course, both Young and Newton were eventually shown to be partially correct, as neither wave nor particle explanations alone can explain the behaviour of light. See e.g.
  7. Young, Thomas (1845). Course of Lectures on Natural Philosophy and the Mechanical Arts. London: Taylor and Walton. 
  8. Truesdell, Clifford A. (1960). The Rational Mechanics of Flexible or Elastic Bodies, 1638–1788: Introduction to Leonhardi Euleri Opera Omnia, vol. X and XI, Seriei Secundae. Orell Fussli. 
  9. "Structures, or Why Things Don't Fall Down" by J. E. Gordon, Penguin Books, 1978.
  10. Young, Thomas (1801). "On the mechanics of the eye". Philosophical Transactions of the Royal Society 91: 23. doi:10.1098/rstl.1801.0004. Bibcode1801RSPT...91...23Y. 
  11. Young, T. (1802). "Bakerian Lecture: On the Theory of Light and Colours". Phil. Trans. R. Soc. Lond. 92: 12–48. doi:10.1098/rstl.1802.0004. 
  12. Young, Thomas (1805). "An Essay on the Cohesion of Fluids". Phil. Trans.: 65. doi:10.1098/rstl.1805.0005. 
  13. Fechner, Gustav Theodor (1878). Ueber den Ausgangswerth der kleinsten Absweichungssumme. S. Hirzel.