All subjects · Physics · WPH12 (Unit 2)

Diffraction and the diffraction grating (with core practical 6)

Diffraction is the spreading of waves through gaps and around obstacles. A diffraction grating uses it to measure wavelength very precisely.

Diffraction and Huygens' construction

Diffraction is the spreading out of waves when they pass through a gap or around an obstacle. It is most noticeable when the gap is about the same size as the wavelength. Huygens' construction: every point on a wavefront acts as a source of secondary wavelets; the new wavefront is the line tangent to (the envelope of) all the wavelets. This explains why waves spread out at the edges of a gap.

The diffraction grating

A grating has many equally spaced slits, a distance d apart. Light from each slit interferes, giving sharp bright maxima at angles θ where

nλ = d sin θ

n is the order (0, 1, 2...). If the grating has N lines per metre, d = 1/N. The highest order possible is the largest whole number below d/λ, because sin θ cannot exceed 1.

Core practical 6

Worked example

Laser light of wavelength 633 nm hits a grating with 300 lines per mm. Find the first-order angle.

  1. d = 1 / (300 x 103) = 3.33 x 10-6 m
  2. sin θ = 633 x 10-9 / 3.33 x 10-6 = 0.190

Answer: θ = 10.9°

Key idea: Diffraction is greatest when the gap is about λ. Grating: nλ = d sin θ, d = 1/(lines per metre), tan θ = x/D on a screen.

Check yourself

  1. A grating has 500 lines per mm. Find the slit spacing d in μm.
    Show answer

    2 μm. d = 1 / (500 x 103 per m) = 2.0 x 10-6 m = 2.0 μm.

  2. Light of wavelength 600 nm falls on a grating with d = 2.0 x 10-6 m. Find the angle of the second-order maximum, in degrees, to 3 s.f.
    Show answer

    36.9 °. sin θ = 2 x 600 x 10-9 / 2.0 x 10-6 = 0.60, so θ = 36.9°.

  3. With a grating of 600 lines per mm, the first-order maximum is at 15.0°. Find the wavelength in nm, to 3 s.f.
    Show answer

    431 nm. d = 1.667 x 10-6 m; λ = d sin θ = 1.667 x 10-6 x 0.2588 = 4.31 x 10-7 m = 431 nm.

  4. Light of wavelength 589 nm falls on a grating with d = 2.0 x 10-6 m. What is the highest order of maximum that can be seen?
    Show answer

    3. d / λ = 3.40, and n must be a whole number with sin θ ≤ 1, so the highest order is 3.

  5. In core practical 6, a grating with 400 lines per mm is 1.50 m from a screen. The first-order maxima are 0.40 m either side of the centre. Find the wavelength in nm, to 3 s.f.
    Show answer

    644 nm. tan θ = 0.40 / 1.50, so θ = 14.9°; d = 2.5 x 10-6 m; λ = 2.5 x 10-6 x sin 14.9° = 6.44 x 10-7 m = 644 nm.

  6. When is diffraction through a gap most noticeable?
    • When the gap is much wider than the wavelength
    • When the gap width is about equal to the wavelength
    • When the wave has a very high amplitude
    • When the gap is much narrower than the wave's amplitude
    Show answer

    When the gap width is about equal to the wavelength. Spreading is greatest when gap and wavelength are similar in size.

  7. What does Huygens' construction assume?
    • Light is made of particles
    • Waves only travel in straight lines
    • Each slit produces a single ray
    • Every point on a wavefront acts as a source of secondary wavelets
    Show answer

    Every point on a wavefront acts as a source of secondary wavelets. The new wavefront is the envelope of all the secondary wavelets.

  8. Which is the most important safety rule in core practical 6?
    • Wear gloves when touching the grating
    • Keep the room brightly lit
    • Never look directly into the laser beam or its reflections
    • Use the highest power laser available
    Show answer

    Never look directly into the laser beam or its reflections. Laser light can damage the retina.

Practise with instant feedback

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Last updated 7 November 2026. Spotted a mistake? Tell us.