Diffraction of Waves

Diffraction is the spreading of a wave as it passes through a gap or around an obstacle. The wave bends around the edges of the gap or obstacle.

Key observations

• Diffraction occurs with all types of waves (sound, light, water).
• The wave speed, frequency and wavelength do not change during diffraction.
• Only the direction of travel changes at the edges.

Effect of gap width

Gap width relative to wavelength	Amount of spreading
Much larger than wavelength	Very little spreading
Similar to wavelength	Maximum spreading (wave spreads out almost in a semicircle)
Much smaller than wavelength	Very little wave energy passes through

Maximum diffraction occurs when the gap width is approximately equal to the wavelength.

Everyday examples

• Sound diffracts around doorways because sound wavelengths (roughly 0.1 m to 10 m) are similar in size to doorways. You can hear someone around a corner even though you cannot see them.
• Light does not noticeably diffract through a doorway because light wavelengths (about $5 \times 10^{-7}$ m) are much smaller than the door opening.
• Water waves diffract through harbour entrances when the entrance width is similar to the wavelength of the waves.

Common errors and how to correct them

Confusing diffraction with refraction. Diffraction is spreading through a gap or around an obstacle in the same medium. Refraction is a change of direction at a boundary between two media due to a speed change.

Saying “the wave gets bigger after diffraction.” The wave spreads out (changes direction at the edges) but does not gain energy. The amplitude actually decreases because the same energy spreads over a wider area.

How to apply this in an exam

State that maximum diffraction occurs when the gap width is similar to the wavelength. For diagrams, show curved wavefronts spreading after the gap. If the gap is much wider than the wavelength, show the wavefronts passing through mostly straight, with slight curving at the edges.