5.5 - Diffraction of Waves

Diffraction and Its Effects

• Diffraction is the spreading of waves as they pass through a slit or around the edge of a barrier. It occurs with water, light and sound waves.
• Water wavefronts passing through a gap in an embankment can change from plane to circular wavefronts as they spread.

Characteristic	Effect of diffraction
Wavelength	Unchanged because wave speed and frequency remain unchanged.
Frequency	Unchanged because the source frequency is unchanged.
Wave speed	Unchanged because the medium is unchanged.
Amplitude	Decreases as energy spreads over a wider region.
Direction	Waves spread from one direction into many directions.

• A larger wave amplitude corresponds to more energy carried by the wave.
• Diffracted laser light forms a spreading pattern after passing through a single slit or pinhole.
• Sound spreads around building corners, so a listener can hear a loudspeaker even when it is hidden from view.

Factors and Wavefront Patterns

Comparison	Effect on spreading
Fixed wavelength: wide slit	Less significant diffraction; wavefronts remain nearly straight in the central region, with bending near the edges.
Fixed wavelength: narrow slit	More significant diffraction; wavefronts become more curved and spread over a wider range of directions.
Fixed slit width: short wavelength	Less significant diffraction.
Fixed slit width: long wavelength	More significant diffraction.

• Diffraction is more significant when the slit width is close to the wavelength.
• In a ripple tank, slit width can be varied while wavelength is fixed. Alternatively, wavelength can be varied by adjusting the generator frequency while slit width is fixed.
• The gap controls the spreading pattern; wavefront spacing remains unchanged before and after the gap in the same medium.

Applications in Daily Life

• Water waves: diffraction through gaps in barriers produces calmer regions suitable for docking ships and water recreation, such as at Kok Beach, Langkawi.
• Light: diffraction effects produce holograms used as security features on debit and credit cards.
• Sound: elephants produce infrasonic waves with long wavelengths, allowing communication over long distances.