Young’s double-slit simulation

Thomas Young’s 1801 experiment showed that light behaves as a wave. Waves passing through two narrow slits spread out and overlap; where they arrive in step they reinforce and where they arrive out of step they cancel.

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What you will see

Straight waves from a paddle reach a barrier with two slits. Behind it two circular wavelets overlap, and calm lines fan out from between the slits where they cancel. A screen on the far side glows with the resulting pattern of bright and dark fringes.

Try this

  1. Trace a calm line from the slits to a dark fringe on the screen.
  2. Count the fringes and compare their spacing with Δy = λD/d.
  3. Orbit the camera to see the wave heights in 3D.

Common misconception

The bright fringes are not images of the two slits. They are produced by interference, so there are many of them, evenly spaced, rather than two.

Exam-style question

Light of wavelength 600 nm passes through slits 0.50 mm apart onto a screen 2.0 m away. What is the fringe spacing?

Δy = λD/d = (600 × 10⁻⁹ × 2.0) / (0.50 × 10⁻³) = 2.4 × 10⁻³ m = 2.4 mm.

Key equations

  • Δy = λD / d
  • d sin θ = mλ

Curriculum

AQA A-level physics 3.3.1–3.3.2, IB Physics C.2–C.3.

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