Double-Slit Interference Lab

Shine light through two narrow slits and you do not get two bright lines. You get a whole strip of alternating bright and dark bands — a pattern that only makes sense if light behaves as a wave.

How the pattern forms

Each slit acts as a new source of spreading waves. Where the two sets arrive in step (crest on crest), they reinforce: bright fringe. Where they arrive out of step (crest on trough), they cancel: dark fringe.

The deciding factor is the path difference — how much further one wave travelled than the other:

• Bright fringe: path difference = whole number of wavelengths, d sin θ = mλ
• Dark fringe: path difference = a half-number of wavelengths, d sin θ = (m + ½)λ

Here d is the slit separation, λ the wavelength, and m an integer (0, 1, 2…). For small angles the fringes are evenly spaced, with a gap of roughly λL/d on a screen at distance L.

What the sliders reveal

• Widen the slit separation d → fringes crowd together (spacing ∝ 1/d).
• Use longer wavelength (red) → fringes spread further apart (spacing ∝ λ).
• Move the screen further away → the pattern stretches.

Try this

• Switch from blue (450 nm) to red (650 nm) and watch the whole pattern expand.
• Close one slit: the fringes vanish, replaced by a single soft blob. Two-slit interference requires two paths.
• Push the slits very close together and the fringes become wide and easy to count.

Why it matters

Thomas Young’s 1801 version of this experiment was the decisive evidence that light is a wave. Two centuries later the same apparatus, fired one photon at a time, still builds up interference — a result at the very heart of quantum mechanics. The technique also underpins X-ray crystallography, which is how we determined the structure of DNA.