Diffraction

Diffraction is a phenomenon that occurs when a wave encounters an obstacle or a gap. Instead of continuing in a straight line, the wave spreads out as it passes through the gap or around the obstacle. This spreading is most noticeable when the size of the gap or obstacle is similar to the wavelength of the wave.

Qualitative Description

• All types of waves (e.g. water, sound, light) can undergo diffraction.
• The extent of diffraction depends on the relationship between the wavelength ($\lambda$) of the wave and the width of the gap ($a$):
  • If $a \gg \lambda$: Little diffraction; most of the wave passes straight through.
  • If $a \approx \lambda$: Significant diffraction; the wave spreads out widely after passing through the gap.
  • If $a \ll \lambda$: The wave is mostly reflected or absorbed; very little passes through.

Demonstrating Diffraction

Water Waves in a Ripple Tank

• A ripple tank can be used to show diffraction with water waves.
• When straight wavefronts (plane waves) approach a gap in a barrier:
  • If the gap is much wider than the wavelength, the waves pass through with little spreading.
  • If the gap is about the same width as the wavelength, the waves spread out in semicircular wavefronts after passing through the gap.

Sound and Light Waves

• Sound waves (with relatively large wavelengths) diffract easily around obstacles and through doorways.
• Light waves (with very small wavelengths) only show noticeable diffraction when passing through very narrow slits.

Key Points

• Diffraction does not change the speed, frequency, or wavelength of the wave; only the direction and shape of the wavefronts are affected.
• The amount of diffraction increases as the gap width decreases relative to the wavelength.