TERM 1 · WAVES, SOUND AND LIGHTTRANSVERSE PULSES AND WAVES
What you must be able to do
Define a pulse, a transverse pulse and amplitude
Apply superposition and draw the resultant of two pulses
Define the wave terms and identify points in phase on a wave diagram
Calculate f, T and v with f = 1/T and v = fλ
Core content · Pulses, amplitude and the medium
Flick a stretched rope once and a pulse runs along it. Each bit of rope only moves up and back down, at right angles to the direction of travel: the pulse is transverse. Only the disturbance and its energy move along.
⚡ MUST MEMORISEPulse: a single disturbance in a medium. Transverse pulse: a pulse in which the particles of the medium move at right angles to the direction of propagation of the pulse. Amplitude: the maximum displacement of a particle from its rest (equilibrium) position.
Core content · Superposition: constructive and destructive interference
When two pulses occupy the same space at the same time, their displacements add: up positive, down negative. Afterwards each continues unchanged in its original direction. A slinky or a ripple tank shows this.
⚡ MUST MEMORISEPrinciple of superposition: the addition of the disturbances of two pulses occupying the same space at the same time. Constructive interference: the phenomenon where the crest of one pulse overlaps with the crest of another to produce a pulse of increased amplitude. Destructive interference: the phenomenon where the crest of one pulse overlaps with the trough of another, resulting in a pulse of reduced amplitude.
Pulse P (4 cm, upward) and pulse Q (3 cm) move towards each other on a slinky. Determine the amplitude of the resultant when they overlap completely if Q is (a) upward and (b) inverted. (4)
(b) ✓: = 1 cm, upward, the direction of the larger pulse
Give size and direction. Two equal pulses, one inverted, cancel completely for that instant.
⚠️ COMMON MISTAKEThe pulses do not bounce off or destroy each other. Drawn after they meet, each keeps its original shape and amplitude and its original direction. Only the overlap frame shows the resultant.
🔥 FREQUENTLY TESTEDThe resultant-pulse multiple choice: two pulses approach; which diagram shows them at full overlap? Add the heights with their signs, and reject any option where a pulse has changed after passing.
Core content · Transverse waves: wavelength, crest, trough and phase
A transverse wave is a stream of pulses. Points in phase have the same displacement and direction of motion, a whole number of wavelengths apart: A and C in Figure 4. A and B are out of phase.
⚡ MUST MEMORISETransverse wave: a succession of transverse pulses. Wavelength (λ): the distance between two successive points in phase. Crest: the highest point on a wave. Trough: the lowest point on a wave.
⚠️ COMMON MISTAKEAmplitude is rest position to crest. Crest to trough is twice the amplitude: 10 cm on Figure 4, and writing it loses the mark.
Core content · Frequency, period and wave speed: v = fλ
Frequency is waves per second (Hz); the period is the time for one, so f = 1/T. In one period a wave advances one wavelength, so v = fλ. The medium sets v: raise f and λ shortens.
⚡ MUST MEMORISEFrequency: the number of wave pulses per second. Period: the time taken for one complete wave pulse. Wave speed: the distance travelled by a point on a wave per unit time.
⚖️ LAWS / FRAMEWORKSData sheet: v = fλ and T = 1/f, with v in m·s−1, f in Hz, λ in m and T in s.
Core content · Reading wave diagrams and worked calculations
Figures 4 and 5 describe the same wave. (a) Write down the amplitude in metres. (b) Write down the wavelength. (c) Name a point in phase with A. (d) Calculate the frequency. (e) Calculate the speed of the wave. (9)
(a) ✓: 5 cm = 0,05 m, rest position to crest
(b) ✓: λ = 3 m, crest A to crest C on Figure 4
(c) ✓: C, one whole wavelength from A
(d) ✓ formula: f = 1/T
(d) ✓ substitution: = 1/0,2 (T = 0,2 s, crest to crest on Figure 5)
(d) ✓ answer: = 5 Hz
(e) ✓ formula: v = fλ
(e) ✓ substitution: = (5)(3)
(e) ✓ answer: = 15 m·s−1
Read λ from the position graph and T from the time graph, never the other way round. Formula and substitution earn two of the three marks even if the arithmetic slips, and a wrong f from (d), used correctly, still earns full marks in (e).
A learner sends 12 complete waves along a rope in 4 s. The distance between a crest and the next trough is 0,35 m. Calculate (a) the period and (b) the speed of the waves. (6)
Step 1: f = 12 waves ÷ 4 s = 3 Hz
(a) ✓ formula: T = 1/f
(a) ✓ substitution: = 1/3
(a) ✓ answer: = 0,33 s
Step 2: Crest to the next trough is half a wavelength: λ = 2 × 0,35 = 0,70 m
(b) ✓ formula: v = fλ
(b) ✓ substitution: = (3)(0,70)
(b) ✓ answer: = 2,10 m·s−1
The trap is the 0,35 m. Substituting it as λ gives 1,05 m·s−1 and loses the substitution and answer marks; the formula mark stands.
⚠️ COMMON MISTAKEUnits and rounding carry marks. A final answer with no unit, or the wrong one, loses the answer mark. Round to at least two decimal places: 0,33 s, not 0,3 s.
Waves of frequency 2,5 Hz travel along a spring at 1,5 m·s−1. Calculate the wavelength. (3)
✓ formula: v = fλ
✓ substitution: 1,5 = (2,5)λ
✓ answer: λ = 0,60 m
Write the data-sheet formula first, then rearrange. Starting from λ = v/f also earns the formula mark.
🧠 ACTIVE RECALL
Which statement correctly defines a transverse pulse?
Which statement correctly defines the amplitude of a wave or pulse?
Which statement describes the wavelength (λ) of a transverse wave?
Which quantity is defined as the time taken for one complete wave pulse to pass a point?
Which statement correctly describes constructive interference of two pulses?
A ribbon is tied to the middle of a long rope. A transverse pulse travels along the rope from left to right. How does the ribbon move as the pulse passes?
At a rugby match the spectators stand up and sit down in turn, so that a “wave” moves round the stadium. Which statement describes this correctly?
A transverse pulse travels from one end of a slinky to the other. What is carried from one end to the other?
After a transverse pulse has passed a point on a stretched rope, where are the particles of the rope at that point?
Which action on a stretched slinky produces a transverse pulse?
Two upward pulses move towards each other on a rope. Which statement is correct while they overlap?
A rope carries an upward pulse and an inverted pulse towards each other. Which statement is correct while they overlap?
Two pulses have passed through each other on a rope and are now apart again. Which statement describes each pulse?
Two equal pulses, one upward and one inverted, overlap and the rope looks flat for an instant. Which statement is correct?
Pulses P and Q meet on a rope. According to the principle of superposition, what is the displacement of the rope at the overlap?
On a slinky, pulse P has an amplitude of 6 cm and is upward. Pulse Q has an amplitude of 2,5 cm and is inverted. What is the resultant pulse when they overlap completely?
A learner at each end of a skipping rope sends a pulse along it. One pulse is 4 cm high and the other is 1,5 cm high, and both are upward. What is the resultant at complete overlap?
Two downward pulses on a stretched spring move towards each other. One has an amplitude of 3 cm and the other 4,5 cm. What is the resultant at complete overlap?
A pulse of amplitude 5 cm, upward, and an inverted pulse of amplitude 8 cm approach each other along a length of rubber tubing. What is the resultant at complete overlap?
Two equal pulses of amplitude 2,8 cm, one upward and one inverted, travel towards each other along a washing line. What is the amplitude of the resultant at the instant of complete overlap?
A wave on a rope has a period of 0,25 s and a wavelength of 0,60 m. What is the speed of the wave?
Exactly 15 waves pass a marker on a spring in 6 s. The distance from a crest to the next trough is 0,20 m. What is the speed of the waves?
A wave of frequency 4,5 Hz travels along a string at 3,6 m·s−1. What is the wavelength of the wave?
Waves travel at 2,0 m·s−1 along a spring and have a wavelength of 0,50 m. What is the period of the waves?
A displacement–time graph shows that one complete oscillation of a particle takes 0,40 s. A displacement–position graph of the same wave shows a wavelength of 1,2 m. What is the speed of the wave?
On a rope, a 2 Hz wave has a wavelength of 0,50 m, so it travels at 1,0 m·s−1. Thato doubles the frequency to 4 Hz on the same rope and says: “The waves now travel at 2,0 m·s−1.” Which evaluation is correct?
Lindiwe measures 0,40 m from a crest to the next trough. She writes λ = 0,40 m and, with f = 2,5 Hz, gets v = 1,0 m·s−1. Which evaluation of her working is correct?
A displacement–position graph shows a crest at +4 cm and a trough at −4 cm. Mandla writes: “Amplitude = 8 cm.” Which evaluation is correct?
A teacher says: “When two equal pulses, one upward and one inverted, overlap, the rope goes flat, so the energy of the pulses is destroyed.” Which evaluation of the statement is correct?
A displacement–time graph shows a period T = 0,25 s and a displacement–position graph shows λ = 3,0 m. Zinhle writes: “f = T = 0,25 Hz, so v = fλ = (0,25)(3,0) = 0,75 m·s−1.” Which evaluation is correct?
A ribbon is tied to the middle of a stretched slinky and a transverse pulse is sent along the slinky from one end. Define a transverse pulse and describe how the ribbon moves as the pulse passes it.
Content: a pulse is a single disturbance in a mediumThe CAPS definition starts with a pulse as ONE disturbance. 'A wave' or 'a movement' does not earn it.
Content: transverse means the particles move at right angles to the direction of propagationThis is what makes the pulse transverse. The examiner listens for 'right angles' or 'perpendicular' to the direction the pulse travels.
Content: the ribbon moves up and down (sideways), at right angles to the slinkyApplying the definition: the ribbon is a particle of the medium, so it moves across the slinky, not along it.
Content: it returns to its rest position; only the disturbance and energy travel alongThe examiner is checking the misconception that the medium travels with the pulse.
A pulse is a single disturbance in a medium;it is transverse when the particles of the medium move at right angles to the direction of propagation of the pulse.As the pulse passes, the ribbon moves up and down (or from side to side), at right angles to the length of the slinky,and then returns to its rest position, because only the disturbance and its energy travel along the slinky, not the ribbon.
Tumi shakes one end of a long skipping rope and makes exactly 20 complete waves in 8 s. The distance from a crest to the next trough is 0,18 m. Calculate (a) the period of the waves and (b) the speed of the waves.
Method: frequency from the count, f = 20 ÷ 8 = 2,5 HzWaves per second is the first step; without it T cannot be found. The examiner wants the division shown.
Method: formula and substitution, T = 1/f = 1/2,5Formula and substitution carry a mark even when the arithmetic slips.
Accuracy: T = 0,40 s with the unitThe answer mark needs the value and the unit; 0,4 s is accepted, 0,4 on its own is not.
Content: crest to the next trough is half a wavelength, λ = 2 × 0,18 = 0,36 mThis is the trap in the question: 0,18 m is NOT the wavelength. Spotting it is the content mark.
Method: formula and substitution, v = fλ = (2,5)(0,36)A wrong f or λ from an earlier step, used correctly here, still earns this mark (consistent accuracy).
Accuracy: v = 0,90 m·s⁻¹ with the unitValue and unit together. A missing or wrong unit (m, s or Hz) loses this mark.
Frequency is the number of waves per second: f = 20 ÷ 8 = 2,5 Hz.Formula and substitution: T = 1/f = 1/2,5so the period is T = 0,40 s.A crest to the next trough is only half a wavelength, so λ = 2 × 0,18 = 0,36 m.Formula and substitution: v = fλ = (2,5)(0,36)so the wave speed is v = 0,90 m·s−1.
Pulse P (amplitude 5 cm, upward) and pulse Q (amplitude 3 cm, inverted) travel towards each other along a rope. (a) Name the type of interference when they overlap and give a reason. (b) Determine the amplitude and direction of the resultant pulse at complete overlap. (c) Describe each pulse after they have passed through each other.
Content: destructive interferenceAn upward pulse meeting an inverted one is the textbook destructive case.
Content: reason, a crest meets a trough (opposite displacements)The reason must mention the pulses being on opposite sides of the rest position.
Method: add with signs, (+5) + (−3)Superposition is an algebraic sum with up positive and down negative; the examiner wants the signs visible.
Accuracy: 2 cm, upward (the direction of the larger pulse)Size and direction are both needed. The answer is 2 cm in the direction of P, the larger pulse.
Content: afterwards each pulse continues unchanged in shape, amplitude and directionThe common mistake is 'they bounce back or destroy each other'; the examiner listens for the pulses passing through.
(a) The pulses undergo destructive interference,because the crest of P meets the trough of the inverted pulse Q, so their displacements work against each other.(b) Resultant = (+5) + (−3)= 2 cm, upward, which is the direction of the larger pulse P.(c) The pulses do not bounce back or destroy each other: each carries on in its original direction with its original shape and amplitude.
Explain to a Grade 8 learner why a Mexican wave can travel all the way round a stadium even though nobody leaves their seat.
Explain to a Grade 8 learner what happens when two pulses meet on a rope, and why the rope can look flat for a moment even though both pulses are still there.
Explain to a Grade 8 learner how to work out how fast a rope wave is travelling, and why shaking the rope faster makes the waves shorter but not faster.
ONE-MINUTE SUMMARY A pulse is a single disturbance; in a transverse pulse the particles move at right angles to the direction of propagation. Pulses that meet add with their signs, then move on unchanged. Read λ off a position graph and T off a time graph, then use f = 1/T and v = fλ, with a unit.
IN THE EXAM
Paper 1, about 12 marks: a Question 1 multiple-choice item and one structured question on a wave diagram.
A 2-mark definition first, then read-offs: amplitude, wavelength, points in phase.
Then f = 1/T and v = fλ: formula, substitution, answer with unit.
Next, sound uses the same v = fλ.
🔒 SOUND
🔒 EM RADIATION
🔒 MAGNETISM
🔒 ELECTROSTATICS
🔒 CIRCUITS
🔒 VECTORS
🔒
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Exam stress is completely normal. Here are a few things that actually work. None of this replaces a doctor or a trusted adult.
Breathe in
Box breathing: 4 counts in, 4 hold, 4 out, 4 hold. Do it four times.
Anxiety
Grounding: name 5 things you see, 4 you hear, 3 you can touch.
Reframe "I am going to fail" into "I have prepared, I will do my best".
Concentration
Work in 25-minute blocks, then a 5-minute break (Pomodoro).
Put the phone in another room while you study.
One task at a time. Multitasking costs more than it saves.
Sleep
Cramming all night backfires: sleep is when the brain locks in memory.
No screens for 30 minutes before bed.
The same bedtime every night keeps the brain sharp.
Study habits
Active recall (testing yourself) beats passive re-reading by far.
Space your study across days; do not cram it all into one.
Practise past papers under time. That is exactly how these notes are built.
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