What this covers
This standard sits within Waves. In a one-to-one lesson we make sure the idea is clear first, then move straight to applying it in the exact way SPM asks, with correct units and full working.
How it is examined
It can appear in Paper 1 (objective) and Paper 2 (structured), and where an experiment applies, in Paper 3. We do not predict which questions appear; we prepare the technique for all of them.
A common mistake
How to study it
Learn the definition precisely, practise one or two SPM-style questions with full working, and link it to the rest of Waves. If it keeps costing marks, a one-to-one lesson fixes exactly that.
Waves · Formulas · Exam Papers
What you need to know
Diffraction is the spreading of waves as they pass through a gap or around the edge of an obstacle. All waves, including water, sound and light waves, can diffract, though the extent of spreading depends strongly on the relationship between the wavelength of the wave and the size of the gap or obstacle.
Diffraction is most noticeable, producing strongly curved wavefronts spreading into the region behind the gap, when the gap size is approximately equal to the wavelength of the wave. If the gap is much wider than the wavelength, the waves pass through with only slight spreading at the edges and continue mostly in their original direction.
When a wave diffracts, its wavelength, frequency and speed remain unchanged since it stays in the same medium; however, the amplitude of the wave decreases because the same amount of energy spreads out over a wider area. In a ripple tank, diffraction is demonstrated by placing two barriers with a narrow gap between them in the path of straight wavefronts and observing how the wavefronts curve around the edges of the gap.
Worked example
Consider a ripple tank set up with straight wavefronts of wavelength 0.02 m approaching a gap between two barriers. When the gap width is set to about 0.02 m, roughly equal to the wavelength, the wavefronts emerging on the far side spread out strongly in a curved, almost circular pattern, filling the region behind the gap.
When the gap width is instead widened to about 0.10 m, much larger than the wavelength, the emerging wavefronts remain mostly straight, spreading only slightly near the edges of the gap while the central portion continues travelling in the original direction. To describe this correctly in a labelled diagram, students should draw the incident straight wavefronts approaching the gap, then show the emerging wavefronts as strongly curved for the narrow gap case and mostly straight with slight edge-curving for the wide gap case, labelling the gap width and noting that the wavelength of the water wave, 0.02 m, is unchanged in both diagrams.
This qualitative comparison is the standard way diffraction is assessed rather than through numerical calculation.
How it is examined
Paper 1 objective questions often test whether diffraction is more noticeable through a narrow or a wide gap relative to wavelength, or ask you to identify which quantity changes during diffraction. Paper 2 structured questions typically present ripple tank diagrams with different gap widths and use command words such as "state" for the condition under which diffraction is most obvious, "describe" for comparing wavefront patterns through narrow and wide gaps, and "explain" for why amplitude decreases while wavelength, frequency and speed remain constant.
"Describe" answers should reference the shape of the wavefronts after passing through the gap, while "explain" answers should link amplitude decrease to the spreading of the same total energy over a larger area. Paper 3 practical work commonly involves a ripple tank with adjustable barriers, where students vary the gap width and sketch the resulting wave patterns.
A common mistake is stating that wavelength changes during diffraction, when in fact only the direction of spreading and the amplitude are affected, not the wavelength, frequency or speed of the wave.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)