Young’s Double-Slit Experiment
In 1801 Thomas Young performed an experiment so simple it could be done on a kitchen table, yet it settled a century-old debate and later became the most iconic demonstration in all of physics. Pass light through two narrow slits, and instead of two bright lines you see a striped pattern of light and dark bands. That pattern can only be explained if light is a wave.
The setup
The experiment is elegantly minimal. Light from a single source shines on a barrier with two closely spaced parallel slits. Beyond the barrier sits a screen. If light were simply a stream of particles travelling in straight lines, you would expect two bright strips on the screen, one behind each slit. Instead you see many alternating bright and dark fringes spread across the screen.
Interference: waves adding and cancelling
The fringes arise from interference. Each slit acts as a fresh source of waves, and these two sets of waves overlap on their way to the screen. Where the crests of one wave meet the crests of the other, they reinforce, producing a bright band, called constructive interference. Where a crest meets a trough, they cancel, producing a dark band, called destructive interference.
- Bright fringe: the two paths differ by a whole number of wavelengths, so the waves arrive in step.
- Dark fringe: the paths differ by half a wavelength, so the waves arrive exactly out of step and cancel.
Interference is the signature of waves. Two streams of particles can only add up, but two waves can cancel each other out, creating dark patches where light plus light gives darkness. That is why the fringes prove light’s wave nature.
Predicting the fringe positions
The geometry of the experiment leads to a precise prediction. Bright fringes appear at angles θ given by:
Here d is the spacing between the slits, λ is the wavelength of the light, and m is a whole number (0, 1, 2, …) labelling each bright fringe. The central fringe sits straight ahead at m = 0, with the others spreading out symmetrically on either side. Measuring the fringe spacing lets you calculate the wavelength of light, which is how Young first estimated it. This wave behaviour complements what happens to light at boundaries, covered in reflection and refraction.
Reading the wavelength from the screen
For small angles, the spacing between adjacent bright fringes on a screen a distance L away simplifies to a tidy result:
This tells us that closer slits or a more distant screen spread the fringes further apart, while longer wavelengths (red light) give wider spacing than shorter ones (blue light). The pattern is a direct visual measurement of something as small as the wavelength of light, a few hundred nanometres.
The quantum twist
Young’s experiment did not just prove light is a wave; it became the gateway to quantum mechanics. When physicists later fired light, or even electrons, one particle at a time, each arrived as a single dot, as a particle would. Yet over many particles, the interference fringes still built up, as if each particle somehow travelled through both slits and interfered with itself.
Even more remarkably, trying to detect which slit the particle goes through destroys the pattern entirely. This refusal to be simply a wave or simply a particle is the heart of wave-particle duality, and the double slit remains the cleanest illustration of quantum strangeness.
Why it still matters
Beyond its conceptual fame, the principle behind the double slit underpins real technology. Diffraction gratings, which use thousands of slits, split light into precise spectra for chemical analysis. Interferometers use the same wave interference to measure distances to a fraction of a wavelength, precise enough to detect gravitational waves rippling through spacetime. A simple experiment from 1801 still echoes through the most advanced laboratories today.
Frequently asked questions
Why are there dark bands between the bright ones?
Dark bands appear where the two waves arrive out of step, with a crest from one slit meeting a trough from the other. They cancel completely, so even though light comes from both slits, the result at those points is darkness.
What happens with just one slit?
A single slit produces a broad central bright region with much fainter bands, caused by diffraction within the single slit. The crisp, evenly spaced interference fringes require two slits so that two wave sources can overlap.
Does the experiment work with particles other than light?
Yes. Electrons, neutrons, atoms, and even large molecules have all produced interference fringes in double-slit experiments, confirming that matter, not just light, exhibits wave behaviour, exactly as de Broglie predicted.
