In 1792 he began to study medicine, intending to follow a friendly and prosperous uncle into his London practice, and in his first year as a medical student in London he published on the physics of the eye. By neat experiments he showed that accommodation (change of focus) is a result of change in the curvature of the lens; at the same time he described and measured astigmatism and in 1801 he devised his three-colour theory of human colour vision. He continued as a medical student, with a full social life, in Edinburgh, Göttingen and Cambridge, and in 1799 set up a practice in London. He was not very successful as a physician, perhaps because (as a friend said) his mind was usually on other matters, and he was not a success as a lecturer at the Royal Institution. During these lectures, when discussing Hooke’s Law in 1802, he gave physical meaning to the constant in that law, which has come to be named as Young’s modulus, E, defined as the ratio stress/strain; here stress is the force per unit area of cross section of a material, which produces a strain measured as (change in length)/(original length). E is a measure of a material’s resistance to change in length, and an average value for natural rubber would be 1X106Nm-2 while for a mild steel E=2X1011Nm-2.

But his major work was on the wave theory of light, which Newton had thought to be corpuscular and Huygens wave-like. Young argued in 1800–04 in favour of the wave theory and supported this by clear and detailed accounts of elegant experiments on interference due to superposition of the waves. The current view entirely supports Young’s interpretation of these effects, while also using a ‘corpuscular’ explanation in terms of photons and quantum theory for such results as photoelectric emission.

From 1814 Young busied himself with his medical practice and with Egyptology, where his major contributions to the interpretation of the Rosetta Stone ultimately revealed the ancient Egyptian system of writing, although at the time others were given more credit for this: in part because his work on it was published anonymously, as the entry on Egypt in a supplement to the Encyclopaedia Britannica for 1819.