Diffraction is the spreading of a wave when it passes through a gap or around an obstacle. How much it spreads depends on the wavelength compared with the size of the gap: the closer the wavelength is to the gap size, the more the wave fans out.
Sound waves in everyday speech have wavelengths of around a metre, similar to the width of a doorway, so they spread strongly and curl around corners. That is why you can hear a friend talking in the next room, or a motorcycle approaching before it comes into view around a bend.
Light has a far shorter wavelength, so it diffracts too little to notice around a doorway, which is why you cannot see around corners the way you can hear around them.
In SPM you should relate the amount of diffraction to wavelength and gap size, and give everyday examples.
Common misconceptions
- Sound bends around corners by bouncing off walls only -> Reflection can help, but the key effect is diffraction, the wave spreading at edges and gaps.
- Light does not diffract at all -> Light diffracts, but its tiny wavelength makes the spreading negligible through ordinary openings.
- Diffraction changes the wave's frequency -> Diffraction changes the direction and spread of a wave, not its frequency.
The physics behind it
Diffraction is the spreading of a wave as it passes through a gap or around the edge of an obstacle. The amount of spreading depends on how the wavelength compares with the width of the gap: the closer they are in size, the more the wave fans out into the region beyond.
The wave equation v = fλ lets us size this up. A fairly low male voice at about 150 Hz has a wavelength of λ = v/f = 340 m s⁻¹ ÷ 150 Hz ≈ 2.3 m, taking the speed of sound as 340 m s⁻¹.
A higher voice at 500 Hz gives λ = 340 m s⁻¹ ÷ 500 Hz = 0.68 m.
Both of these wavelengths are comparable to the width of an ordinary gap between buildings or a gate about a metre across, so sound diffracts strongly and reaches your ears even when the source is out of sight. Light, with a wavelength of less than a millionth of a metre, is far too small compared with such gaps, so it barely diffracts and travels in near-straight lines.
See it in daily life
Walking through a housing area, you often hear a hawker calling out or the recorded jingle of a passing bread van before you round the corner and actually see the vehicle. The brick wall of the corner house blocks your view completely, yet the sound arrives clearly.
This happens because the sound waves reach the edge of the wall and spread outward into the shaded region behind it, bending into the lane where you stand. Their wavelength, around a metre or so, is similar to the size of the gaps and edges they pass, so they diffract generously.
Light from the same van has such a tiny wavelength that it does not bend around the corner at all, which is why your ears get the news before your eyes do.
The effect is even stronger for the deep rumble of distant thunder, whose long wavelengths curl easily around houses and hills, so the sound seems to come from everywhere at once rather than one clear direction.
How this comes up in SPM
In Paper 2 this idea is examined with command words such as relate, explain, describe and compare. You may be asked to relate the degree of diffraction to wavelength and gap size, to explain why sound diffracts more than light, or to describe a ripple-tank demonstration of diffraction.
Within the same Waves chapter, diffraction sits between refraction, which changes a wave's direction as its speed changes, and interference, which needs diffracted waves from two gaps to overlap. It rests on the fundamentals of waves, where v = fλ links speed, frequency and wavelength.
A frequent task is to describe how a water wave changes as it passes through a wide gap and then a narrow gap in a ripple tank, and to relate the wider spreading at the narrow gap to the wavelength. Draw the wavefronts curving at the edges, keep the wavelength unchanged through the gap, and use v = fλ with correct units when a numerical wavelength is required.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)