The Reluctant Polymath

Thomas Young and the Wave Particle Coherence

Inside a darkened room in London in November 1803, physician Thomas Young covered a sunlit window with a thick shutter, pierced a tiny pinhole in the screen with a fine needle, and used a small looking-glass to angle the pencil-thin sunbeam horizontally across the room. Into the middle of that narrow ray—just one-thirtieth of an inch wide—he inserted a thin slip of paper playing card edgewise, slicing the single beam of sunlight into two grazing streams that passed on either side of the card and struck the opposite wall.

For ninety-nine years since Isaac Newton had published his Opticks in 1704, British natural philosophy had treated Newton's corpuscular theory of light as unassailable orthodoxy. Light, Newton had argued, consisted of microscopic material pellets shot from luminous bodies in straight lines; if light were a wave like sound, it would bend around corners into every shadow. Under Newton's particle model, splitting a sunbeam with a narrow strip of card should have cast a sharp dark shadow down the center of the wall flanked by two bright patches of light.

Instead, Young watched the shadow zone behind the playing card break into a delicate fan of alternating bright and dark colored stripes. Most striking of all, the exact dead center of the geometric shadow was always a bright white fringe. When Young held a small screen against one edge of the card to block just one of the two grazing sunbeams, the entire pattern of alternating bands vanished instantly—even though the light passing the other edge of the card had not been touched. Adding a second stream of light to the first had caused specific regions of the wall to turn completely dark.

Young had arrived at optics through anatomy and acoustics rather than orthodox Newtonian mechanics. Born into a Somerset Quaker family in 1773, he had mastered Greek, Latin, Hebrew, Syriac, Arabic, and Persian before studying medicine in London, Edinburgh, and Göttingen. As medical student in 1793, dissecting the eye of a recently slaughtered ox, Young proved that the eye focuses on near and distant objects not by elongating the entire eyeball, but by ciliary muscles changing the curvature of the elastic crystalline lens. A few years later, studying the acoustic beats of organ pipes and ripple tanks while lecturing at the Royal Institution, he recognized that two overlapping waves cancel into silence or still water whenever the crest of one wave meets the trough of another.

Applying that acoustic geometry to his split sunbeam, Young calculated the exact geometric path difference $\Delta L$ between light rays traveling from two slits or edges separated by distance $d$ to a point on a screen at angle $\theta$:

Whenever $m$ is an integer ($0, 1, 2, \dots$), the two wave crests arrive in phase to form a bright band; whenever $m$ is a half-integer ($m + \frac{1}{2}$), crest meets trough in destructive interference, producing darkness. Measuring the tiny spacing between his fringes with a micrometer, Young made the first empirical measurement of the wavelength $\lambda$ of visible light, showing that extreme red light undulates at roughly 36,000 waves per inch (0.70 micrometers) while extreme violet light oscillates at 60,000 waves per inch (0.42 micrometers).

That same structural eye for periodic patterns guided Young across solid mechanics—where he defined Young's modulus of elasticity—and into ancient cryptography. Beginning in the summer of 1814 during his annual holidays in Worthing, Young examined copies of the Rosetta Stone, the black granodiorite stele uncovered at Rashid in 1799 bearing a 196 BCE royal decree in Greek, Demotic, and Egyptian hieroglyphs. By statistically matching recurring sign clusters in the Demotic text against the 14 lines of hieroglyphs and isolating the royal cartouches containing the foreign Greek name Ptolemaios, Young proved that Demotic was a cursive form of hieroglyphs and deciphered the first phonetic hieroglyphic signs. Although Henry Brougham's vicious 1803 attacks in the Edinburgh Review silenced British support for Young's wave theory for over a decade, Augustin-Jean Fresnel's independent diffraction experiments in Paris in 1818 confirmed Young's phase equation down to the thousandth of a millimeter.