4.4WavesCore

Diffraction

Waves don’t just travel in straight lines — they spread out whenever they pass through a gap or around an obstacle. Pass light through thousands of tiny, evenly-spaced slits instead of just one, and that spreading becomes precise enough to measure a wavelength to remarkable accuracy.

40 min Video by Physics Online Module 4.4 Waves

Diffraction Gratings — A Level PhysicsWatch the full walkthrough before the notes below.

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What you'll be able to do

Describe diffraction as the spreading of a wave passing through a gap or around an obstacle
Explain how the amount of diffraction depends on the gap width relative to the wavelength
Use the diffraction grating equation $d sin θ = nλ$
Explain why a grating with more lines per mm gives more widely-separated maxima

Diffraction through a single slit

$Diffraction$ is the spreading out of a wave as it passes through a gap or around an obstacle. The amount of spreading is most significant when the gap width is comparable to the wavelength of the wave — a gap much wider than the wavelength produces very little noticeable spreading, while a gap comparable to (or smaller than) the wavelength produces substantial diffraction.

Tip — This is why sound (long wavelength) diffracts noticeably around a doorway, letting you hear someone in the next room, while light (very short wavelength) shows almost no visible diffraction through the same doorway.

The diffraction grating

A $diffraction grating$ consists of a very large number of narrow, evenly-spaced slits. Light passing through produces sharp, well-defined bright maxima at specific angles, where light from every single slit arrives in phase and interferes constructively.

d sin θ = nλ

d = slit spacing, θ = angle to the maximum, n = order number (0, 1, 2, …), λ = wavelength.

1Slit spacing is the reciprocal of lines per unit length:

d = \frac{1}{500 per mm} = \frac{1}{500 \times 1 0 ^{3} per m}

Answer

d = 2.0 \times 1 0^{- 6}

Tip — A grating with MORE lines per mm has a SMALLER slit spacing d, which — from d sinθ = nλ — spreads the maxima out to LARGER angles, giving better separation between wavelengths.

Equation recap

d sin θ = nλ

Diffraction grating equation.

Common mistakes to avoid

Believing all gaps produce equally strong diffraction, regardless of size.

Significant diffraction requires the gap width to be comparable to the wavelength — a gap much larger than the wavelength produces very little spreading.

Using slit spacing in millimetres directly in the grating equation.

Always convert slit spacing to metres before substituting — "lines per mm" must first be converted into a spacing d in metres.

Assuming a grating with fewer lines per mm gives better wavelength separation.

MORE lines per mm means a SMALLER d, which spreads maxima to larger, more easily distinguished angles — better resolution comes from more lines per mm, not fewer.

Key takeaways

Diffraction is most significant when gap width is comparable to wavelength.
A diffraction grating produces sharp maxima where dsinθ = nλ.
More lines per mm (smaller d) spreads maxima to larger angles, improving wavelength resolution.

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Diffraction

40 min Video by Physics Online Module 4.4 Waves