Young's Double-Slit Experiment: Light as a Wave

In 1801, Thomas Young directed a narrow beam of sunlight through two closely spaced slits cut in a card, then projected the result onto a screen a short distance away. He expected, if light consisted of particles, to see two bright stripes. Instead he saw many: alternating bright and dark bands across the screen. The pattern could not be explained by particles. It was the signature of waves interfering with each other — the same pattern produced when two sets of ripples meet on a pond. The experiment changed the direction of physics, was confirmed by two centuries of refinement, and then, when performed with single electrons in the 1960s, became the central demonstration of quantum mechanics. It is the same setup, performed at two very different scales, producing the same impossible-looking result.

The State of Optics Before Young

By 1801, the nature of light had been debated for well over a century, and the debate had a clear front-runner.

Isaac Newton had argued in his 1704 Opticks that light consists of a stream of particles, which he called "corpuscles." His reasoning was partly theoretical — particles traveling in straight lines naturally explained why light casts sharp shadows — and partly empirical. Newton had observed the colors of thin films (the rainbow patterns in soap bubbles and oil slicks) and had produced a mathematical description of them, known as "Newton's rings," that seemed to fit a corpuscular theory with the addition of what he called "fits of easy reflection and easy transmission." He was aware of interference-like phenomena but interpreted them within a corpuscular framework.

Newton's authority in physics was immense. By the early 19th century, his corpuscular theory of light was the dominant view among British and French natural philosophers, even though Christiaan Huygens had proposed a wave theory of light in 1678 and Leonhard Euler had further developed wave arguments in the 18th century. The wave theory had problems: if light were waves, what medium did they travel through? How could waves travel through the vacuum of space, as light from the sun clearly did? The proposed medium — the ether — was hypothetical and had never been detected. The corpuscular theory seemed more parsimonious.

Thomas Young's double-slit experiment produced a result that the corpuscular theory could not explain, that the wave theory predicted exactly, and that anyone could observe directly. It did not immediately overturn Newton's corpuscular theory — the established view had too much institutional weight — but it opened the fault line that eventually collapsed it.

Source: Britannica — "Thomas Young: Double-Slit Experiment, Biography, and Facts" (britannica.com/biography/Thomas-Young)

Who Was Thomas Young in 1801

Thomas Young (1773–1829) — English physician, physicist, and scholar. He contributed to the wave theory of light, deciphered Egyptian hieroglyphs (the Rosetta Stone), described astigmatism in optics, and established the modulus of elasticity now called Young's modulus. He was widely considered one of the most intellectually capable generalists of his era.

Young was a child prodigy who could read at age two and had studied six languages by age 14. He trained as a physician — medicine was the career that provided his income throughout his life — but his intellectual interests ranged across physics, biology, linguistics, and Egyptology. He presented his double-slit results to the Royal Society in 1801 in a lecture titled "On the Theory of Light and Colours," and published a fuller account in his 1807 A Course of Lectures on Natural Philosophy and the Mechanical Arts.

Young was not well received by his British contemporaries. Henry Brougham, writing anonymously in the Edinburgh Review in 1803, attacked Young's wave theory with contempt, suggesting that Young had produced nothing but a refinement of Huygens' century-old ideas and that his experiments proved nothing. Young was stung by the criticism and responded sharply, but the British physics community was slow to accept the wave interpretation. The final vindication came from France: Augustin-Jean Fresnel developed a rigorous mathematical theory of wave optics in the 1810s–1820s that predicted diffraction and interference patterns quantitatively, and Siméon Denis Poisson — a committed corpusculist — derived from Fresnel's theory the prediction that there should be a bright spot at the center of the shadow behind a circular disc (the "Poisson bright spot" or "Arago spot"). When Dominique Arago confirmed the spot experimentally in 1818, the wave theory was effectively established.

Source: Wikipedia — "Young's interference experiment" (en.wikipedia.org/wiki/Young's_interference_experiment)

The Setup: Sunlight, Card, and Screen

Young's original apparatus was elegantly simple. He admitted sunlight into a darkened room through a small hole in a window shutter, producing a narrow beam. He placed a card with two small holes (or slits) in the beam. He observed the light from the two holes on a white screen or wall placed a short distance behind the card.

The precise dimensions of Young's original apparatus are not definitively recorded, but his 1807 publication describes slits approximately 1/30 of an inch (about 0.85 mm) apart, with the screen placed several feet behind the card. The exact geometry produced visible interference fringes — alternating bright and dark bands — spaced by roughly 1–2 mm, which are visible with the naked eye against a light background.

The key feature of the setup is that the two slits act as two coherent sources of light — two sources emitting waves that maintain a fixed phase relationship with each other, because both slits are illuminated by the same wavefront from the original pinhole. (If the two slits were lit by independent, incoherent sources — like two separate candles — the interference pattern would wash out because the relative phase of the two sources would vary randomly.)

Coherent light sources — light sources that maintain a constant phase relationship with each other, producing stable interference patterns. Two slits illuminated by the same monochromatic source are coherent; two separate light bulbs are not.

In Young's original experiment, sunlight (which contains all visible wavelengths) produced colored fringes — a central white band flanked by colored fringes, because different wavelengths produce different fringe spacings. With monochromatic light (a single wavelength), the fringes are uniformly spaced bright and dark bands.

Source: Lumen Learning Physics — "Young's Double Slit Experiment" (courses.lumenlearning.com/suny-physics/chapter/27-3-youngs-double-slit-experiment/)

The Physics: Why Waves Produce Stripes

The interference pattern — the alternating bright and dark bands — is exactly what is expected when two sets of waves overlap. Understanding why requires thinking about wave addition.

Constructive interference — when two waves arrive at the same point with their peaks and troughs aligned (in phase), they add together: the resulting amplitude is larger than either wave alone. At these points on the screen, the light is brighter.

Destructive interference — when two waves arrive at the same point with one wave's peak aligned with the other's trough (out of phase by half a wavelength), they cancel: the resulting amplitude is zero. At these points, the screen is dark.

The screen is placed at different distances from the two slits, depending on the position on the screen. At the center of the screen, directly in front of the midpoint between the two slits, the distances from the two slits are equal. Light from both slits travels the same total distance and arrives in phase. Constructive interference: bright central band.

Moving slightly away from the center, one slit is farther than the other. When the path difference — the difference in distance traveled by light from the two slits — equals half a wavelength, the two waves arrive out of phase. Destructive interference: dark band. When the path difference equals one full wavelength, the waves are back in phase. Bright band. And so on across the screen, producing a series of bright and dark bands whose spacing depends on the wavelength and the geometry.

Think of it like two stones dropped simultaneously into still water. Each creates circular ripples spreading outward. Where the two sets of ripples meet, they add: where crest meets crest, the water surface is higher; where crest meets trough, the water surface is undisturbed. The double-slit experiment is the optical version of this ripple pattern, with light replacing water and the screen replacing the water surface.

The mathematical relationship is: bright fringes appear at positions y = mλL/d, where m is the fringe order (0, ±1, ±2, ...), λ is the wavelength, L is the distance from slits to screen, and d is the slit separation. Young used this relationship to calculate the wavelength of light from the spacing of the fringes he observed — producing the first measurements of the wavelength of visible light. He found wavelengths ranging from about 400 nm for violet light to about 700 nm for red light — values consistent with modern measurements (violet: ~380–450 nm, red: ~620–750 nm).

Source: Evident Scientific / Molecular Expressions — "Thomas Young's Double Slit Experiment" (evidentscientific.com/en/microscope-resource/tutorials/doubleslitwavefronts)

Why Particles Cannot Produce This Pattern

The corpuscular theory of light predicts a completely different result from the double-slit experiment. If light consists of particles traveling in straight lines, each particle passes through either the left slit or the right slit. The particles that pass through the left slit spread slightly (due to diffraction, which Newton acknowledged) and produce a bright region on the screen roughly behind the left slit. Particles through the right slit produce a bright region roughly behind the right slit. The total pattern on the screen should be the sum of these two single-slit patterns: a broad bright central region with no structure, or at most two overlapping blobs.

What is definitely not predicted by classical particles is the dark bands. If particles pass through either slit and land on the screen, how can there be regions on the screen that receive fewer particles when both slits are open than when only one slit is open? A particle traveling through the left slit has no way of "knowing" whether the right slit is open, and no mechanism by which the presence of an open right slit can reduce the number of left-slit particles arriving at a particular point.

This is the logical force of the double-slit experiment: it is not merely that the pattern looks wave-like, but that the pattern cannot be produced by particles whose trajectories are independent. The experiment forces a choice: either the entities traveling through the slits are waves (or behave like waves), or they have some property that allows their individual trajectories to be influenced by whether remote slits are open — which is not a property of classical particles.

Source: Double-slit experiment — Wikipedia (en.wikipedia.org/wiki/Double-slit_experiment)

A Timeline of the Double-Slit Experiment and Its Extensions

Year	Experimenter	Development
1678	Christiaan Huygens	Proposes wave theory of light; explains reflection and refraction but lacks interference
1704	Isaac Newton	Publishes Opticks; corpuscular theory dominant
1801	Thomas Young	Double-slit experiment with sunlight; observes interference fringes; measures wavelength of light
1807	Thomas Young	Publishes full account in A Course of Lectures on Natural Philosophy
1818	Augustin Fresnel	Mathematical wave theory; Arago spot confirms wave prediction — corpuscular theory effectively abandoned
1864	James Clerk Maxwell	Electromagnetic wave theory of light; light is oscillating electric and magnetic fields
1905	Albert Einstein	Photoelectric effect explains light as photons — particle behavior returns
1924	Louis de Broglie	Proposes matter waves; electrons have wavelength λ = h/mv
1961	Claus Jönsson	First double-slit experiment with electrons; produces interference fringes
1974	Pier Giorgio Merli	Double-slit with electrons fired one at a time; single electrons build up the interference pattern
1989	Akira Tonomura	Clean single-electron double-slit demonstration; each electron leaves a single dot, pattern emerges statistically
2013	Various groups	Double-slit with molecules as large as C₆₀ (buckminsterfullerene) and larger organic molecules

Source: Quantum Zeitgeist — "The Double-Slit Experiment: Quantum Wave-Particle Duality" (quantumzeitgeist.com/the-double-slit-experiment-the-most-important-physics-experiment-ever/)

The Quantum Twist: Single Electrons and the Measurement Problem

Young's 1801 experiment established the wave nature of light. The quantum version of the same experiment, developed through the 20th century, is far stranger.

In 1961, German physicist Claus Jönsson performed the double-slit experiment using electrons instead of light — machining slits 300 nm wide in copper foil and irradiating them with a 40 keV beam of electrons. The electrons, which are particles with definite mass and charge, produced an interference pattern on the detector screen. This was not entirely surprising in light of de Broglie's 1924 proposal that matter has a wave nature described by a wavelength λ = h/mv (where h is Planck's constant, m is mass, and v is velocity). Electrons have a de Broglie wavelength small enough to produce visible interference when passing through slits of appropriate size.

The result that made the double-slit experiment famous in quantum mechanics was what happened when electrons were fired through the slits one at a time. If only one electron is in the apparatus at a time, there is no possibility of one electron interfering with another. And yet, as each electron arrives at the detector screen, it makes a single localized dot — behaving like a particle on arrival. As more and more single-electron dots accumulate, the pattern they form is an interference pattern. Each individual electron lands at a random position, but the statistical distribution of thousands of single-electron events builds up the same stripes that a continuous beam produces.

This is the wave-particle duality problem at its starkest. Each electron arrives as a localized particle — one dot, one location. But its choice of where to land follows a probability distribution determined by wave interference, as if the electron as a whole had passed through both slits simultaneously and interfered with itself. The electron does not split: it arrives as a single entity. But its trajectory is governed by interference between possibilities.

Wave function — in quantum mechanics, a mathematical function that describes the probability amplitude for finding a quantum system in each possible state. The wave function for an electron in the double-slit experiment extends through both slits simultaneously; the interference of the wave function from the two slits determines where the electron is likely to land.

Source: MIT Physics — "Famous Double-Slit Experiment Holds Up When Stripped to Its Quantum Essentials" (news.mit.edu/2025/famous-double-slit-experiment-holds-when-stripped-to-quantum-essentials-0728)

The Measurement Problem: What Happens When You Watch

The deepest implication of the quantum double-slit experiment emerges when you try to determine which slit each electron passed through.

If you place a detector near one of the slits to record which slit each electron passes through, the interference pattern disappears. The electrons now land in two blobs — one behind each slit — exactly as classical particles would. The act of measuring which slit the electron used changes the pattern from interference to two-blob.

This is not a disturbance from a clumsy measurement device. Even a measurement that barely interacts with the electron — designed to cause the minimum possible disturbance — destroys the interference, as long as the measurement provides enough information to distinguish which slit was used. The relevant quantity is not the physical disturbance but the information obtained: if the which-slit information is available (even in principle, even without anyone looking at it), the interference vanishes.

This behavior is one of the central puzzles of quantum mechanics. The electron behaves as a wave — spread through both slits, interfering with itself — as long as there is no record of which slit it passed through. The moment such a record exists, it behaves as a classical particle that passed through one slit. What "observation" or "measurement" means at a fundamental level, and why obtaining information about a quantum system changes its behavior, is the measurement problem — arguably the deepest unresolved conceptual issue in physics.

Source: Brilliant — "Double-Slit Experiment" (brilliant.org/wiki/double-slit-experiment/)

What People Get Wrong About Young's Experiment

The most common misconception is that the double-slit experiment was about quantum mechanics from the start. Young's 1801 experiment had nothing to do with quantum mechanics — the concept did not exist. It was a demonstration that light was a wave, in the classical sense, against a prevailing corpuscular theory. The quantum twist came 160 years later, when the experiment was performed with single particles.

A related error is saying that light is "both a wave and a particle" as if this is a stable dual property light possesses simultaneously. More precisely, light (and matter) has a quantum nature that produces wave-like behavior in some experimental contexts (interference) and particle-like behavior in others (the photoelectric effect, individual photon detection). The apparent duality is a consequence of trying to describe quantum systems using the concepts of classical physics. Within quantum mechanics, the entity is described by a wave function; what behaves like a "wave" or a "particle" depends on what is measured and how.

Another misconception is that the experiment proves light has no particle nature. Einstein's 1905 paper on the photoelectric effect showed that light behaves as discrete quanta (photons) in other contexts, and this interpretation won him the Nobel Prize in 1921. Light is not purely a wave any more than it is purely a particle. The double-slit experiment established wave behavior; the photoelectric effect established particle behavior; quantum electrodynamics, developed from the 1940s onward, provides the unified description.

For how quantum mechanics applies to atomic structure, see how atoms work. For the broader history of how quantum mechanics developed from these foundational experiments, see the history of quantum mechanics.

FAQs

Q: Why do the dark bands appear in the double-slit pattern?
A: Dark bands appear where light waves from the two slits arrive with opposite phases — one wave's peak coincides with the other's trough — and cancel each other out. This cancellation (destructive interference) reduces the light intensity to zero at those points. For a particle model of light, there is no mechanism that could produce this cancellation, because particles from one slit cannot "know" about particles from the other slit to cancel them.

Q: How did Young measure the wavelength of light from the fringe spacing?
A: Young used the geometric relationship between fringe spacing, slit separation, and the distance from slits to screen. Bright fringes appear at positions where the path difference from the two slits is an integer number of wavelengths. By measuring the fringe spacing (Δy), the slit separation (d), and the distance to the screen (L), he could calculate the wavelength as λ = Δy × d / L. He found wavelengths ranging from roughly 400 nm (violet) to 700 nm (red), consistent with modern measurements.

Q: What did Newton's rings have to do with the wave theory?
A: Newton's rings are interference patterns produced by the thin air gap between a convex lens and a flat glass plate. Light reflects from the top and bottom surfaces of the thin air film; at each point, the reflected waves interfere constructively or destructively depending on the film thickness, producing colored concentric rings. Newton observed and described these rings but interpreted them within his corpuscular framework as "fits of easy reflection." Young showed that the same patterns arise naturally and quantitatively from the wave theory of interference, without requiring any additional assumptions — making the wave theory simpler and more predictive.

Q: Why was Young not immediately accepted by British scientists?
A: Newton's authority in Britain was enormous, and the corpuscular theory was the established view. Young's wave theory required accepting that light was a transverse wave — oscillating perpendicular to its direction of travel — which raised immediate questions about what medium carried the wave. The ether remained hypothetical. Additionally, Young's mathematical presentation was not as rigorous as Fresnel's later work, and his responses to critics were sometimes more polemical than persuasive. The French wave theory community, led by Fresnel and Arago, provided the mathematical and experimental confirmation that ultimately convinced the physics community.

Q: What is the largest object that has been shown to produce interference in a double-slit experiment?
A: Researchers have progressively demonstrated interference with increasingly massive objects. In 2019, a team at the University of Vienna showed interference with molecules containing over 2,000 atoms and molecular weights above 25,000 atomic mass units — far larger than the carbon-60 (buckminsterfullerene) molecules first used in earlier experiments. The de Broglie wavelength of these massive, slow-moving molecules is extremely small, requiring very fine slits and a very sensitive detection setup, but the interference pattern appears. There is no current theoretical upper limit at which interference should disappear, though practical experimental difficulties scale rapidly with mass.

Q: How does the double-slit experiment relate to modern technology?
A: The wave behavior of electrons demonstrated by the double-slit experiment underlies electron microscopy, which uses electrons rather than photons to image structures smaller than the wavelength of visible light. Electron wavelengths at the energies used in electron microscopes are below 0.01 nm — far shorter than the ~400–700 nm of visible light — allowing resolution of individual atoms. The interference and diffraction of electrons also underlie X-ray crystallography (which uses the wave nature of X-rays to determine molecular structure) and the design of optical coatings, anti-reflection coatings on lenses, and interference filters used in telecommunications and scientific instruments.

The Takeaway

Young's double-slit experiment has been performed for 225 years with sunlight, monochromatic lasers, single photons, electrons, neutrons, atoms, and molecules. The result is always the same: interference fringes that can only be explained by wave behavior, combined with discrete arrivals at detectors that look like particles. The experiment forced physics to abandon a purely corpuscular theory of light in 1801, forced it to accept a quantum theory of matter in the 20th century, and continues to anchor the deepest unresolved questions about the nature of measurement in quantum mechanics. Young himself had no idea what he was starting. He set up a card with two holes, let sunlight through, and observed stripes on a wall — the kind of observation that, if you know what you are looking at, changes everything you thought you knew about the nature of light.

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