5 high-yield, syllabus-aligned questions on Waves, light & sound, covering 15 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Define4 marks
Define wavelength, frequency and amplitude, and state the wave equation.
TopMarks model answer
The wavelength is the distance between two successive points on a wave that are in phase, for example two successive crests. The frequency is the number of complete waves produced, or passing a fixed point, in one second. The amplitude is the maximum displacement of a particle of the medium from its rest position. The wave equation is v = fλ, where v is the speed of the wave, f the frequency and λ the wavelength.
Suggested marking guide
1Wavelength as the distance between successive points in phase
2Frequency as waves per second
3Amplitude as the maximum displacement
4The wave equation
Why it scores
Two words in these definitions are doing all the work. Wavelength needs IN PHASE, because the distance between any two points is not a wavelength. Amplitude needs MAXIMUM, because the displacement changes continuously and only its maximum value is the amplitude. Drop either word and the definition is no longer true.
A common incomplete answer
“Wavelength is the distance between two crests, and frequency is the number of waves per second.”
What it costs: Three marks of four. Frequency is credited. The wavelength definition omits “successive”, amplitude is not defined, and the wave equation is not given.
Key ideas to include: in phase, successive crests, complete waves per second, maximum displacement, v = fλ.
State3 marks
State three differences between light waves and sound waves.
TopMarks model answer
Light is a transverse wave whereas sound is a longitudinal wave. Light is an electromagnetic wave and can travel through a vacuum, while sound is a mechanical wave that requires a material medium and cannot travel through a vacuum. Light travels very much faster than sound; sound travels at about 330 m s⁻¹ in air.
Suggested marking guide
1Transverse versus longitudinal
2Travels through a vacuum versus requires a medium
3Great difference in speed
Why it scores
The vacuum difference is the one worth understanding rather than memorising: sound is a vibration OF the medium, so with no medium there is nothing to vibrate. That is why an electric bell ringing inside an evacuated jar can be seen but not heard — a favourite examiner’s demonstration question.
A common incomplete answer
“Light is a transverse wave and sound is a longitudinal wave.”
What it costs: Two marks of three. A correctly paired difference, but only one of the three the question asked for.
Key ideas to include: transverse, longitudinal, electromagnetic, mechanical wave, material medium, vacuum.
Calculate3 marks
Explain why an echo is heard, and calculate the distance of a cliff from a man who claps his hands and hears the echo 3 s later. [speed of sound in air = 330 m s⁻¹]
TopMarks model answer
An echo is heard because the sound wave is reflected from a large hard surface, in this case the cliff, and returns to the observer. The sound travels to the cliff and back, so the total distance covered is 2d, where d is the distance to the cliff. Total distance = speed x time = 330 x 3 = 990 m. Therefore 2d = 990 m and d = 495 m.
Suggested marking guide
1An echo is a reflected sound
2The sound covers twice the distance
3Correct answer with the unit
The working, line by line
1Total distance travelled = speed x time = 330 x 3 = 990 m — This is the distance the SOUND covers, not the distance to the cliff.
2The sound goes to the cliff and back, so total distance = 2d — This single line is the mark that separates 990 m from 495 m. Write it down before you divide.
32d = 990 → d = 495 m — Halve, and state the unit.
Why it scores
Every echo calculation contains the same hidden factor of two, and every year candidates give 990 m. The physics is not hard — the discipline of writing “the sound travels there and back, so the total distance is 2d” is what earns the mark and prevents the error.
A common incomplete answer
“Distance = speed x time = 330 x 3 = 990 m.”
What it costs: Two marks of three. The arithmetic is right but it gives the total path of the sound, not the distance to the cliff. The reflection is never explained and the factor of two is never applied.
Key ideas to include: echo, reflection, total path, twice the distance.
State2 marks
State the laws of reflection of light.
TopMarks model answer
The first law of reflection states that the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. The second law states that the angle of incidence is equal to the angle of reflection.
Suggested marking guide
1All three lie in the same plane
2Angle of incidence equals angle of reflection
Why it scores
There are TWO laws and the first is the one that gets forgotten, because it feels obvious. It is not obvious and it is not free: the coplanarity of the incident ray, the reflected ray and the normal is a separate physical statement, and it is a separate mark.
A common incomplete answer
“The angle of incidence is equal to the angle of reflection.”
What it costs: One mark of two. The second law is correct and credited, but the first law — the coplanarity of the rays and the normal — is missing.
Key ideas to include: incident ray, reflected ray, normal, same plane, angle of incidence.
Explain3 marks
Explain the term refraction and state two of its effects that can be observed in everyday life.
TopMarks model answer
Refraction is the change in direction of a wave as it passes obliquely from one medium into another of different optical density, and it is caused by the change in the speed of the wave in the two media. A stick partly immersed in water appears bent at the surface. A swimming pool appears shallower than it really is, and the sun remains visible for a short time after it has actually set below the horizon.
Suggested marking guide
1Change in direction on passing between media
2Caused by a change in the speed of the wave
3A valid everyday effect
Why it scores
The middle mark is the physics. Refraction is not caused by the boundary; it is caused by the wave travelling at a different speed in the second medium. A wave meeting the boundary along the normal still changes speed but does not change direction — which is why the word “obliquely” belongs in the definition.
A common incomplete answer
“Refraction is the bending of light when it enters another medium.”
What it costs: Two marks of three. The definition is credited, but the cause — the change in speed — is missing, and no everyday effect is given.
Key ideas to include: change in direction, optical density, change in speed, obliquely, apparent depth.
Practise Waves, light & sound on real questions
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