Light transverse EM; sound longitudinal
Light is a transverse electromagnetic (EM) wave — field oscillations perpendicular to the energy transfer. It needs no medium and travels through a vacuum at m/s. Sound is a longitudinal mechanical wave — molecules oscillate parallel to travel, as compressions and rarefactions; it needs a medium and cannot cross a vacuum (space is silent). Speed of sound in air ≈ 340 m/s, about a million times slower than light.
Reflection, refraction, n, TIR
Law of reflection: angle of incidence = angle of reflection (from the normal). Refraction: light changes speed and direction on entering a new medium. Refractive index (dimensionless, ≥ 1). Total internal reflection (TIR): past the critical angle (denser to less dense medium), all light reflects back; . Uses: optical fibres and prisms. Mini-example: → , so .
Sound: echo, hearing range, pitch, loudness
Echo = reflected sound; distance , where is the round-trip time — the factor 2 is essential. Human hearing: 20 Hz to 20 000 Hz (below = infrasound, above = ultrasound). Ultrasound uses: medical scanning (foetal), sonar depth-finding. On a CRO, the period is read from the x-axis and . Pitch rises with frequency; loudness rises with amplitude (maximum displacement from equilibrium).
Drawn from real examiner reports.
Echo: forgetting to divide by 2
The commonest lost mark in echo questions is not dividing by 2. Sound travels to the reflector AND back, so the recorded time is the round trip: , not . Rearranged for speed with a one-way distance and round-trip time : . Always show the ÷2 step: "total distance , one-way ". Skipping it caps you at 2 of 3 marks.
November 2024 Paper 2P Q8(b) — examiner report stated "virtually all candidates scored either 2 or 3 marks for the speed calculation, even if they forgot to use double the given distance to account for the echo." This confirms that forgetting the factor of 2 is the dominant error pattern on echo questions.
Sound in space / light needs air
Both marks need distinct points: (1) light is a transverse EM wave that travels through a vacuum; (2) sound is a longitudinal mechanical wave that needs a medium and cannot cross a vacuum. "Both travel through air" or "sound travels in space" loses both. Calling sound transverse is a trap — its particles move parallel to travel.
November 2024 Paper 2P Q8(d) — examiner report noted "candidate explanations tended to be simplistic which would not score marks given this was the last item on the paper," indicating many students gave surface-level answers without the required causal detail.
Human hearing range boundaries
The accepted human hearing range is 20 Hz to 20 000 Hz; both bounds must be right. Errors: "20 kHz to 20 000 kHz" (a ×1000 slip on the top), or dropping units. Ultrasound is sound above 20 000 Hz — defined by FREQUENCY, not loudness, not "very loud" sound. Also define amplitude as maximum displacement from equilibrium, not "wave height".
June 2024 Paper 2P Q5(b) — examiner report noted "large numbers of candidates wanted to include a comment about column E [above 20 000 Hz] without appropriate qualification." The correct upper limit is 20 000 Hz; frequencies above this are ultrasound, which humans cannot hear.
Reading amplitude to find frequency
On a CRO trace, frequency comes from the time (x) axis only — count the squares for one cycle to get , then . Reading the wave HEIGHT (y-axis amplitude) to find frequency scores zero: amplitude sets loudness, not pitch. A louder sound is a TALLER wave (same spacing); a higher-pitched sound has cycles CLOSER together (same height if loudness is fixed).
Ultrasound vs infrared
Both appear in medical/imaging contexts, but they are different waves. Ultrasound is SOUND above 20 000 Hz — mechanical, longitudinal, needs a medium (used in scanning and sonar). Infrared (IR) is an EM wave — transverse, travels through a vacuum (used in remote controls and thermal imaging). Do not call ultrasound electromagnetic or say it travels through a vacuum.
Measure angles from the normal
In reflection and refraction, angles are measured from the normal (the line perpendicular to the surface), NOT from the surface. For , is the angle in the less dense medium (e.g. air) and in the denser medium. Measuring from the surface, or swapping and , gives the wrong . Entering a denser medium, light bends TOWARDS the normal.
Echo: show the ÷2 step
Echo scaffold: (1) write (or ) to show the factor of 2; (2) substitute with the halving on its own line; (3) add the unit (m or m/s). Stating "÷2 because sound goes to the wall and back" secures the mark. Never put the round-trip time into .
Oscilloscope: get T from the x-axis
Oscilloscope scaffold: (1) count the squares for one complete cycle on the x-axis; (2) multiply by the time-per-division to get in seconds; (3) apply . Read the TIME axis, never the amplitude, for frequency. Convert ms → s before taking the reciprocal.
"Show that": show the working
On a "show that" period/frequency question the answer is given, so marks are for METHOD. State (or ), convert units (ms → s), substitute and evaluate to confirm the value. A bare answer with no working scores zero. Be ready to work or .
Compare light and sound: three points
For "describe a difference between light and sound" give distinct points: (a) wave type — transverse (light) vs longitudinal (sound); (b) medium — light needs none, sound needs one; (c) speed — light is far faster (≈ vs 340 m/s). One point alone caps the marks.
Wave speed (applies to both light and sound):
Period and frequency:
Reference speeds:
Define a transverse wave and give one example.
A factory siren makes a loud sound. The sound travels toward a large wall and an echo is heard 0.80 s later. The speed of sound in air is 340 m/s. Calculate the distance from the siren to the wall.
