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Every Class 12 student learns the formula for fringe width in Young's double-slit experiment. Fewer know that Young was a practising physician who also worked on how the eye focuses, on elasticity and on Egyptian hieroglyphs, and that he made his case for light waves at a time when Newton's authority made the idea unpopular in Britain. This post sets out what is reliably known about him, what his experiment shows, and how to use it to measure the wavelength of light.
Who was Thomas Young?
The MacTutor biography from the University of St Andrews gives his birth as 13 June 1773 in Milverton, Somerset, and his death as 10 May 1829 in London. He was elected a Fellow of the Royal Society on 19 June 1794, while still a medical student. He studied medicine at Edinburgh and then at Göttingen, where he passed his examination on 30 April 1796, and later at Emmanuel College, Cambridge.
His interests were unusually wide. NCERT's chapter on wave optics describes him as an English physicist, physician and Egyptologist who worked on problems from the structure of the eye and the mechanism of vision to the decipherment of the Rosetta Stone. MacTutor adds that Young's modulus, the measure of a material's stiffness you meet in Class 11, was first introduced in his 1807 Course of Lectures on Natural Philosophy.
What did Young argue about light?
In late 1801 Young read a paper to the Royal Society, On the theory of light and colours, published in 1802. It opens with an unusual claim for a physics paper: "Nor is it absolutely necessary in this instance to produce a single new experiment; for of experiments there is already an ample store". His aim was to show that a wave theory, which earlier thinkers had proposed, could explain a great many known facts. MacTutor notes that the same paper put forward the theory of three-colour vision.
In November 1803 he read a second paper, Experiments and calculations relative to physical optics, published in 1804. According to MacTutor, it presented a wave theory that explained interference patterns and described how he had found the wavelengths of different colours using diffraction.
NCERT dates his interference experiment to 1801 and says it "firmly established" that light is a wave. For the next forty years or so, many experiments on interference and diffraction could be explained only by assuming that light was a wave.
How did Young's arrangement make two matching sources?
Two separate lamps cannot produce a steady fringe pattern, because their phases change randomly and independently. NCERT explains Young's solution: he lit two pinholes, S1 and S2, very close together, from a single pinhole that was itself lit by a bright source. Any change in the original source appears in both S1 and S2 at once, so the two stay "locked" in phase. They are coherent, and the waves from them produce stable bright and dark fringes on a screen.
Bright fringes appear where the path difference is a whole number of wavelengths, at positions x = nλD/d, where D is the distance to the screen and d is the separation of the two sources. The spacing between neighbouring bright fringes, the fringe width, is β = λD/d.
Worked example: measuring the wavelength of light
This is the measurement that makes Young's experiment so useful. Suppose two slits 0.50 mm apart are 1.25 m from a screen, and you count 11 bright fringes spread over 15.0 mm.
Count the gaps, not the fringes. 11 bright fringes have 10 gaps between them, so β = 15.0 mm ÷ 10 = 1.50 mm.
Rearrange the formula. λ = βd / D.
Convert to metres. β = 1.50 × 10^-3 m, d = 0.50 × 10^-3 m, D = 1.25 m.
Calculate. λ = (1.50 × 10^-3 × 0.50 × 10^-3) / 1.25 = 0.75 × 10^-6 / 1.25 = 6.0 × 10^-7 m = 600 nm.
That is orange light, close to the "about 0.6 µm" that NCERT gives for yellow light. A ruler-scale measurement has given you a length of less than a thousandth of a millimetre.
Which mistakes come up in exam questions?
Dividing by the number of fringes instead of gaps. With 11 fringes you have 10 spacings.
Mixing millimetres and metres. Convert everything to metres before substituting.
Moving d and D around. Fringes get wider when the screen moves further away (larger D) and narrower when the slits move apart (larger d).
Forgetting the medium. If the apparatus is in water, the wavelength in the medium is smaller, so the fringes are narrower.
For a full problem set, see our guide to Young's double-slit problems.
What can a student take from Young's way of working?
Reread the evidence you already have. Young's 1801 paper argued that existing experiments were enough to support the wave theory, if read correctly.
Use more than one subject. As a physician he studied how the eye focuses, and the same 1801 paper that argued for light waves also proposed three-colour vision.
Turn an idea into a measurement. Interference did not just support waves; it let him estimate the wavelength of each colour.
If the idea of coherent sources still feels slippery, it helps to see two waves add. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read, and interference from two sources is a good example of one. Next in this series, read how Augustin Fresnel turned the wave theory into a mathematical one.
Redo the worked example with 9 bright fringes spread over 12.0 mm, and check whether you get a wavelength of 600 nm.
