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The Double-Slit Experiment Explained
When light of one colour passes through two narrow slits close together, the screen behind does not show two bright lines. It shows a row of evenly spaced bright and dark stripes called interference fringes. Thomas Young first did this in 1801, and it was the experiment that convinced physicists that light is a wave.
Why the stripes appear
Each slit acts as a new source of waves. At any point on the screen, the waves from the two slits arrive having travelled slightly different distances: the path difference Δ. If Δ is a whole number of wavelengths (0, λ, 2λ, …), crest meets crest and the light adds up: a bright fringe (constructive interference). If Δ is a half-number (λ/2, 3λ/2, …), crest meets trough and they cancel: a dark fringe (destructive interference).
The key formulas
For slit separation d, screen distance L and wavelength λ, a point at height y on the screen has path difference Δ ≈ d·y/L (when L is much larger than d and y). So:
- Bright fringes at ym = mλL/d (m = 0, ±1, ±2, …)
- Dark fringes at y = (m + ½)λL/d
- Fringe width β = λL/d: the spacing between neighbouring bright fringes
So the fringes spread out for longer wavelengths (red more than blue), for a more distant screen, and for slits that are closer together.
Worked example
Green light, λ = 532 nm, passes through slits d = 0.25 mm apart onto a screen L = 1.5 m away. Fringe width β = λL/d = (532 × 10⁻⁹ × 1.5) / (0.25 × 10⁻³) = 3.19 × 10⁻³ m ≈ 3.2 mm.
Single slit vs double slit
A single slit also spreads light out (diffraction), giving a broad central band with weaker side bands. The first dark band is at sin θ = λ/a for slit width a. In a real double-slit experiment you see both effects together: fine two-slit fringes inside the wider single-slit envelope.
One photon at a time
If the light is made so dim that photons pass one by one, each lands at a single random spot, yet after thousands of photons the same fringes build up. The fringes vanish if anything records which slit each photon went through. This is one of the central puzzles of quantum mechanics.
Frequently asked questions
What does the double-slit experiment prove?
It shows that light behaves as a wave: waves from the two slits interfere, adding up in some places and cancelling in others, which produces bright and dark fringes.
What is the formula for fringe width?
Fringe width β = λL/d, where λ is the wavelength, L the distance from the slits to the screen and d the separation between the slits.
What happens to the fringes if you close one slit?
The interference fringes disappear. Only the smooth single-slit diffraction pattern of the open slit remains, because there is no second wave to interfere with.
Why do red fringes look wider than blue ones?
Fringe width is proportional to wavelength. Red light has a longer wavelength (about 650 nm) than blue (about 450 nm), so its fringes are further apart.