Young's Double-Slit Experiment Explained Explained

In 1801, Thomas Young performed one of the most elegant experiments in the history of physics. He shone light through two narrow slits onto a screen — and instead of two bright bands, he saw a pattern of alternating bright and dark fringes. This could only happen if light behaved as a wave, with the two beams interfering constructively and destructively. Young's double-slit experiment settled, for a century, the question of whether light was a wave or a particle. (Quantum mechanics later showed it is both — but that understanding came only in the 20th century.)

How the Experiment Works

A coherent light source (one with consistent phase relationship — a laser is ideal) illuminates two narrow slits separated by distance a. Each slit acts as a secondary source of Huygens wavelets. The two sets of waves spread out and overlap in the region beyond the slits.

At any point on the screen, the waves from the two slits have travelled different distances — this difference is the path difference δ. Whether they arrive in phase or out of phase determines whether that point is bright or dark.

Constructive interference (bright fringe): waves arrive in phase — path difference is a whole number of wavelengths:

Destructive interference (dark fringe): waves arrive perfectly out of phase — path difference is a half-integer number of wavelengths:

The Fringe Spacing Formula

For a screen at distance D from the slits (with D ≫ a), the fringe spacing (distance between adjacent bright or dark fringes) is:

This formula allows the wavelength of light to be measured precisely: rearranging, λ = aΔy/D. By measuring the slit separation a, the screen distance D, and the fringe spacing Δy with a ruler, Young measured the wavelength of light for the first time.

Fringe spacing increases when:

• λ increases (longer wavelength light → wider fringes: red wider than blue)

• D increases (screen further away → wider fringes)

• a decreases (slits closer together → wider fringes)

Worked Examples

Example 1: Finding wavelength

Two slits separated by a = 0.5 mm = 5 × 10⁻⁴ m are placed 2.0 m from a screen. Fringe spacing Δy = 2.4 mm = 2.4 × 10⁻³ m. Find the wavelength of light.

600 nm — orange-red visible light. ✓

Example 2: Fringe spacing with blue light

Blue light (λ = 450 nm) passes through slits 0.3 mm apart, screen 1.5 m away.

Example 3: Path difference to dark fringe

Light of wavelength 500 nm illuminates double slits. What path difference produces the 2nd dark fringe?

Conditions for Clear Interference Fringes

Coherence: the two sources must be coherent — maintaining a constant phase relationship. A laser provides perfect coherence; ordinary light sources must have light from a single source divided by the two slits (so they remain phase-correlated). Incoherent sources produce overlapping, randomly-phased patterns that wash out the fringes.

Monochromatic light: a single wavelength gives sharp, clearly separated fringes. White light produces overlapping fringe patterns for each wavelength, creating coloured fringes near the centre with the central white maximum, then a blurred pattern further out.

Slit width: slits must be narrow enough to produce diffraction (spreading of light through the slit), allowing overlap of the two beams. Wider slits produce a single-slit diffraction envelope that modulates the double-slit pattern.

Young's Experiment and Quantum Mechanics

Young's experiment became even more profound in the 20th century. When performed with single photons — fired one at a time — the interference pattern still builds up on the screen over thousands of photon detections. Each photon hits the screen at a single point (particle-like), yet the statistical distribution over many photons creates the interference pattern (wave-like). The same experiment with single electrons, neutrons, and even atoms gives the same result.

This is the core of wave-particle duality: each quantum object interferes with itself, following a probabilistic wave description, but is detected as a discrete particle. And if you add a detector to determine which slit each photon/electron goes through, the interference pattern disappears — the act of measurement collapses the wave behaviour. Young's simple experiment thus sits at the very heart of quantum mechanics.

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