What this covers
Interference of Waves is part of the Waves chapter. We teach it the way it is tested: the concept in plain English, then a worked example, then a question the student tries while the teacher checks the method.
Formulas you may need
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
Interference occurs when two waves overlap in the same region of space, combining according to the principle of superposition, which states that the resultant displacement at any point is the sum of the individual displacements of the overlapping waves. Constructive interference happens when a crest meets a crest, or a trough meets a trough, producing a resultant displacement larger than either wave alone.
Destructive interference happens when a crest meets a trough, producing a resultant displacement that is reduced, or completely cancelled if the amplitudes are equal. Clear, observable interference patterns require two coherent sources, meaning sources that produce waves of the same frequency and wavelength with a constant phase difference.
In double-slit experiments with water or light waves, the fringe spacing is related to the slit separation and the distance to the screen by λ = ax/D, where a is the separation between the two coherent sources in metres (m), x is the fringe separation, the distance between adjacent bright or dark fringes, in metres (m), and D is the distance from the sources to the screen in metres (m). This formula must be memorised for calculations.
Worked example
In a double-slit interference experiment using light of unknown wavelength, the slit separation a is 0.50 mm, which is 0.50 × 10⁻³ m, the distance to the screen D is 1.5 m, and the measured fringe separation x is 1.8 × 10⁻³ m. Using the formula λ = ax/D, the wavelength is calculated as λ = (0.50 × 10⁻³ m × 1.8 × 10⁻³ m) / 1.5 m = 6.0 × 10⁻⁷ m.
This value corresponds to visible light, showing how measuring the fringe pattern on a screen allows the wavelength of the light source to be determined without measuring it directly. Students must convert all quantities to metres before substituting into the formula, since mixing millimetres and metres is a frequent source of numerical error.
The working should be set out in clear steps: state the formula, list the known values with units, substitute carefully, and present the final answer to an appropriate number of significant figures with the unit m clearly shown, since the wavelength here is extremely small and standard form is essential for clarity.
How it is examined
Paper 1 questions test recognition of constructive and destructive interference from diagrams showing overlapping wavefronts, and may require identifying coherent sources from a description. Paper 2 structured questions commonly involve a double-slit diagram with command words such as "define" for coherent sources or interference, "state" for the conditions producing constructive or destructive interference, and "calculate" for finding wavelength, fringe separation or slit distance using λ = ax/D, with every value substituted in consistent units, usually metres.
"Calculate" answers lose marks if units are inconsistent, for example mixing millimetres for a with metres for D without converting first. Paper 3 practical work may involve a ripple tank with two coherent point sources or dippers vibrating together, where students observe and sketch the pattern of nodal and antinodal lines.
A common mistake is confusing which points show constructive interference on a diagram, forgetting that it occurs where crest meets crest or trough meets trough, not simply where two wavefronts cross; another is failing to convert units consistently before using the interference formula.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)