Diffraction

Section: Superposition  |  Syllabus: Cambridge AS Level Physics (9702)

What is Diffraction?

Waves travel in straight lines when unobstructed. When waves pass the edge of an obstacle or pass through a gap between obstacles, they spread out. This spreading of waves is called diffraction.

Diffraction The spreading out of waves when they pass the edge of an obstacle or pass through a gap between obstacles into regions of geometrical shadow. Universal Property

Diffraction occurs for all wave types: water waves, sound, light, microwaves, and other EM waves. It requires only a single wave source-coherence is not needed.

Huygens' Explanation

Every point on a wavefront acts as a secondary source of wavelets. The new wavefront is the sum of these wavelets. At a gap, only the points within the gap act as sources, causing the wave to spread out.

FIG 8.11: Huygens' Construction for Diffraction

Show: incident plane wavefront approaching a gap in a barrier, secondary sources (dots) within the gap, curved wavelets emanating from each secondary source, resulting diffracted wavefront spreading into the geometrical shadow region. Label the shadow areas on either side of the direct path.

Conditions Affecting Diffraction

The extent of diffraction depends on the ratio of wavelength to gap width (or obstacle size).

FIG 8.12: Diffraction Through Different Gap Widths

Show three cases using wavefronts: (a) λ << a (gap much wider than wavelength): waves pass through mostly straight with slight edge curving, (b) λ ≈ a (gap comparable to wavelength): waves spread out significantly in semicircular pattern, (c) λ >> a (gap much smaller): very little transmission. Label λ (wavelength), a (gap width), and direction of wave travel.

Effect of Gap Width

Maximum Diffraction Condition:

Maximum spreading occurs when: gap width ≈ wavelength

i.e., when a ≈ λ

where:

Effect of Wavelength

For a fixed gap width:

Why This Matters

This explains why we can hear sounds around corners (sound has wavelengths of metres, comparable to doorway widths) but cannot see around corners (light has wavelengths of nanometres, much smaller than typical gaps).

Diffraction Around Obstacles

Diffraction also occurs when waves encounter an obstacle. The waves bend around the edges of the obstacle and partially enter the "shadow" region behind it.

Experimental Demonstrations of Diffraction

1. Water Waves in a Ripple Tank

Plane waves generated by a rectangular paddle are directed towards a barrier with an adjustable gap.

FIG 8.13: Ripple Tank Diffraction

Show two scenarios: (a) Wide gap (a >> λ): plane wavefronts passing through with slight edge curvature, (b) Narrow gap (a ≈ λ): waves spreading out in semicircular pattern. Label: incident plane waves, gap width a, diffracted waves, geometrical shadow regions.

Observations:

2. Diffraction of Light Through a Single Slit

Monochromatic light (e.g., laser) passes through a narrow slit onto a distant screen.

FIG 8.14: Single-Slit Diffraction Pattern

Show: laser → narrow slit → screen. On screen, show intensity pattern: wide, bright central maximum, then alternating dark minima and progressively dimmer secondary maxima on both sides. Include intensity vs position graph below showing the characteristic pattern with central peak much wider and taller than side peaks.

Observations:

Effect of slit width:

Wave Evidence

Single-slit diffraction demonstrates the wave nature of light. Particles would not spread into shadow regions.

3. Sound Diffraction Around Obstacles

Sound waves diffract readily because their wavelengths (typically 0.017 m to 17 m for audible sound) are comparable to everyday objects and gaps.

Observations:

Practical example: When music is playing in another room, you may hear the bass notes more clearly than the treble notes through a partially open door, because the longer wavelength bass sounds diffract more around the door edge.

Diffraction Pattern Intensity

In single-slit diffraction of light, the intensity distribution shows characteristic features:

Note on Single Slit vs Double Slit

Single-slit diffraction produces a pattern with a dominant central maximum and rapidly diminishing secondary maxima. This is distinct from double-slit interference, which produces multiple equally-spaced fringes (covered in section 8.3).

Common Exam Mistakes

Avoid These Errors
  1. Confusing diffraction with refraction: Diffraction is the spreading of waves at gaps/obstacles. Refraction is the change in direction when waves enter a different medium at an angle.
  2. Thinking diffraction requires two sources: Diffraction occurs with a single wave source passing through a gap. Interference patterns require two or more coherent sources.
  3. Getting the gap-wavelength relationship wrong: Maximum diffraction occurs when gap width ≈ wavelength, not when gap width >> wavelength.
  4. Believing diffraction changes wavelength: Diffraction does not change the wavelength or frequency of a wave; only the direction of propagation changes.
  5. Forgetting diffraction occurs for all waves: All waves exhibit diffraction, including electromagnetic waves, sound waves, and water waves.

Concept Check Questions

Interactive revision notes, videos and practice questions load below.

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