---
title: "Thomas Young (scientist)"
chunk: 2/3
source: "https://en.wikipedia.org/wiki/Thomas_Young_(scientist)"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:07:28.656184+00:00"
instance: "kb-cron"
---

"After some information concerning his affairs, and some instructions concerning the hierographical papers in his hands, he said that, perfectly aware of his situation, he had taken the sacraments of the church on the day preceding. His religious sentiments were by himself stated to be liberal, though orthodox. He had extensively studied the Scriptures, of which the precepts were deeply impressed upon his mind from his earliest years; and he evidenced the faith which he professed; in an unbending course of usefulness and rectitude."

== Research ==

=== Wave theory of light ===

In Young's own judgment, of his many achievements the most important was to establish the wave theory of light set out by Christiaan Huygens in his Treatise on Light (1690). To do so, he had to overcome the century-old view, expressed in the venerable Newton's Opticks, that light is a particle. 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 additional experiments, including the predecessor of the double-slit experiment, he demonstrated interference in the context of light as a wave.

Young, speaking on 24 November 1803, to the Royal Society of London, began his now-classic description of the historic experiment:

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.
In his subsequent paper, titled Experiments and Calculations Relative to Physical Optics (1804), Young describes an experiment in which he placed a card measuring approximately 0.85 millimetres (0.033 in) 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 in front or behind the narrow strip so as to prevent the light beam from striking one of its edges caused the fringes to disappear. This supported the contention that light is composed of waves.
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 Augustin-Jean Fresnel.

=== Young's modulus ===

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.
However, the first use of the concept of Young's modulus in experiments was by Giordano Riccati in 1782—predating Young by 25 years.
Furthermore, the idea can be traced to a paper by Leonhard Euler published in 1727, some 80 years before Thomas Young's 1807 paper.
Young's modulus relates the stress (compression or decompression) in a body to its associated strain (fractional change in length); that is, stress = E × strain, for a uniaxially loaded specimen. Young's modulus is independent of the quantity under investigation; that is, it is an intensive property of the material. Young's Modulus allowed, for the first time, the 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."

=== Vision and colour theory ===
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; and in his lectures he presented the hypothesis, afterwards developed by Hermann von Helmholtz, (the Young–Helmholtz theory), that colour perception depends on the presence in the retina of three kinds of nerve fibres. 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.