Diffraction grating simulation

A diffraction grating has hundreds of slits per millimetre. Light from all of them interferes, so each wavelength only emerges at a few precise angles, which makes gratings far better than prisms for measuring spectra.

Interactive, free and runs in your browser. Part of the Optics simulations.

What you will see

A white beam from a lamp hits a ruled grating. The straight-through zeroth order stays white; each higher order fans out into a continuous rainbow and lands on a curved screen as bright bands. Higher orders are dimmer and wider, and disappear once sin θ would exceed 1.

Try this

  1. Double the lines per millimetre and watch the first order spread out.
  2. Count how many orders appear at 300 and at 1200 lines per mm.
  3. Compare the width of the first and second order spectra.

Common misconception

More slits do not move the bright orders. The angles depend only on the slit spacing d; adding slits makes each order sharper and brighter.

Exam-style question

A grating has 600 lines per mm. At what angle is the first-order maximum for 500 nm light, and how many orders can be seen?

d = 1/600 mm = 1.67 μm. sin θ = λ/d = 0.500/1.67 = 0.30, so θ ≈ 17.5°. The highest order has m ≤ d/λ = 3.33, so orders up to m = 3 appear on each side.

Key equations

  • d sin θ = mλ
  • d = 1 / N

Curriculum

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

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