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
When a wave meets a boundary between two media, or hits a surface it cannot pass through, part or all of its energy bounces back; this is reflection. The law of reflection states that the angle of incidence equals the angle of reflection, where both angles are measured from the normal, an imaginary line drawn perpendicular to the reflecting surface at the point where the wave strikes it.
This law applies to all types of waves, including water waves, sound waves and light waves. During reflection, the frequency, wavelength and speed of the wave remain unchanged because the wave stays in the same medium; only the direction of travel changes.
This can be demonstrated using a ripple tank, where straight water waves are directed at a flat, rigid barrier placed at an angle. The incident wavefronts and reflected wavefronts can be observed and their angles measured relative to the normal, confirming that they are equal.
Diagrams of reflection must clearly show the incident ray, reflected ray, normal line drawn as a dashed line, and both angles labelled correctly on the same side of the normal as their respective rays.
Worked example
Consider a ripple tank experiment where straight wavefronts approach a flat barrier placed at an angle to the direction of wave travel. A ray is drawn perpendicular to the wavefronts to represent the direction of travel, and a normal line is drawn perpendicular to the barrier at the point of incidence.
If the incident ray makes an angle of 40° with the normal, then by the law of reflection, the reflected ray also makes an angle of 40° with the normal, on the opposite side. The labelled diagram should show the barrier as a straight line, the normal as a dashed line perpendicular to the barrier, the incident wavefronts approaching at the given angle, and the reflected wavefronts leaving at the same angle, with the wavelength of the incident and reflected waves drawn equal in spacing since the wavelength does not change.
Students should state clearly that the angle of incidence, 40°, equals the angle of reflection, 40°, and that the speed and frequency of the wave are unaffected by the reflection.
How it is examined
Paper 1 objective questions frequently test the law of reflection directly, asking you to calculate a missing angle when one angle and the normal are given, or to identify correctly labelled diagrams of reflection. Paper 2 structured questions often require you to sketch reflected wavefronts in a ripple tank diagram, using command words such as "state" for the law of reflection, "sketch" for drawing the reflected wavefronts and normal, and "describe" for explaining what stays constant during reflection.
"Sketch" answers are assessed on whether the normal is drawn correctly, whether the angles on either side are equal, and whether wavefront spacing is preserved. Paper 3 practical work commonly uses a ripple tank with a plane barrier to observe reflection of straight or circular wavefronts, requiring careful measurement of angles using a protractor.
A frequent mistake is measuring the angle of incidence or reflection from the barrier surface itself instead of from the normal, which gives an incorrect angle; another is assuming the wave slows down or changes wavelength upon reflection, which is incorrect since only direction changes.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)