Q: What does IP Combined Science Notes (Lower Sec, Year 1-2): 09) Waves, Light & Sound cover? A: Use wave speed equations, draw ray diagrams, and interpret sound investigations covering frequency and amplitude.
Wave behaviour explains ripples on water, colour from prisms, and the pitch of your voice. Work with both qualitative diagrams and quantitative calculations.
These notes align with MOE's Lower Secondary Science syllabus themes commonly taught in IP Sec 1-2, and act as a bridge into upper-secondary Physics, Chemistry, and Biology.
Status: MOE Lower Secondary Science syllabus (current release) checked 2025-11-30 - scope unchanged; remains the reference for these combined science notes.
The core idea is simple: Waves transfer energy without transferring matter overall.
Use it as a working check: Use wave speed, frequency, and wavelength together. For light, draw the normal first. For sound, link pitch to frequency and loudness to amplitude.
Then go one layer deeper: Example: if wave speed stays constant and frequency doubles, wavelength halves because more wave crests pass each second.
Learning targets
Define transverse and longitudinal waves with particle motion diagrams.
Apply v=fλ to calculate wave speed, frequency, or wavelength.
Draw ray diagrams for reflection, refraction, and dispersion.
Interpret sound intensity and frequency data, including oscilloscope traces.
Transverse waves: particles oscillate perpendicular to wave direction (light, water surface). Longitudinal waves: particles oscillate parallel to wave direction (sound).
Particle-motion checkpoint
Before naming the wave type, separate the direction the wave travels from the direction one particle vibrates.
Wave type
Wave travels
Particle motion
Good mental picture
Trap to avoid
Transverse wave
Along the direction energy is transferred.
Up and down, or side to side, at right angles to the travel direction.
A rope pulse moving along the rope while each point of rope moves up and down.
Saying the particles travel all the way with the wave.
Longitudinal wave
Along the direction energy is transferred.
Back and forth parallel to the travel direction.
Air particles compress and spread out as sound passes.
Drawing crests and troughs and forgetting compressions and rarefactions.
Worked check: if a sound wave moves from left to right through air, each air particle vibrates left and right about its own position. The sound energy moves across the room, but the same air particle does not travel across the room with it.
Misconception check: waves transfer energy. The particles of the medium usually vibrate around fixed positions instead of being carried along overall.
Calculation-choice checkpoint
Before substituting numbers, identify what the question gives you and what must stay the same. This keeps wave-speed questions from becoming formula hunting.
Question clue
Use this relationship
What to reason before calculating
Speed, frequency, and wavelength appear.
v=fλ
If the wave medium is unchanged, wave speed is usually treated as constant.
Frequency and period appear.
T=f1
Higher frequency means a shorter time for one complete wave.
A wave reaches a distance after a time.
d=vt
Convert units first so speed, distance, and time match.
Frequency changes but wave speed is constant.
fλ=constant
Frequency and wavelength change in opposite directions.
Common trap: amplitude does not appear in v=fλ. A louder sound has a larger amplitude, but that does not mean the wave speed is larger in the same medium.
2 Wave calculations
Worked example - Ripple tank
Ripples have wavelength 1.5cm and frequency 12Hz.
v=fλ=12×1.5×10−2=0.18,m⋅s−1.
If wave speed remains constant and frequency doubles, wavelength halves.
Sound travel time
Speed of sound in air ≈340m⋅s−1. Lightning thunder delay of 4.5s implies distance:
d=vt=340×4.5=1.53×103,m.
Sound travel-time checkpoint
Before using d=vt, decide whether the sound travels one way or makes a round trip. Echo questions often hide this extra step.
Situation
Sound path
Distance to calculate
Common trap
Thunder heard after lightning
Sound travels from lightning to observer once.
d=vt.
Dividing by 2 even though there is no reflection.
Echo heard from a wall or cliff
Sound travels to the surface and back.
Distance to wall =2vt.
Using vt as the one-way distance.
Two observers hear a sound at different times
Compare two one-way travel times.
Difference in distance =vΔt.
Treating the later sound as an echo without a reflecting surface.
Worked check: an echo returns 0.80s after a clap. The sound has travelled to the wall and back, so the total path is 340×0.80=272m. The wall is half that distance away: 136m.
Misconception check: only halve the distance when the time includes an outgoing and returning sound path.
3 Light behaviour
Before drawing any light ray diagram, set up the construction marks first. This prevents angle labels from being measured from the surface instead of from the normal.
Diagram task
Draw first
Then check
Reflection from a plane mirror
Mirror line, normal line, incident ray
Incident angle and reflected angle are measured from the normal and are equal.
Refraction into a denser medium
Boundary, normal line, incident ray
The refracted ray bends towards the normal.
Refraction into a less dense medium
Boundary, normal line, incident ray
The refracted ray bends away from the normal.
Rectangular glass block
Normal at entry and normal at exit
The emergent ray should be parallel to the incident ray if the block faces are parallel.
Reflection
Law of reflection: angle of incidence = angle of reflection. Use normal line for diagrams.
Plane mirror image checkpoint
For a plane mirror question, construct the image location before drawing the reflected ray. The image forms the same perpendicular distance behind the mirror as the object is in front of it.
Diagram step
What to draw
Why it matters
Common trap
1. Mark the object point
Label the object point in front of the mirror.
The image position is measured from this point.
Measuring from the eye instead of from the object.
2. Draw a perpendicular to the mirror
Use a dashed construction line at right angles to the mirror.
Plane-mirror distance is measured along the normal direction.
Measuring the image distance along a slanted ray.
3. Place the image point
Put the image the same distance behind the mirror.
The image is virtual because rays only appear to come from behind the mirror.
Drawing the image on the mirror surface.
4. Link the eye to the image
Draw a straight line from the eye to the image, then mark the reflected ray from mirror to eye.
This gives the correct point where the reflected ray leaves the mirror.
Guessing the bounce point before locating the virtual image.
Worked check: if a candle is 8cm in front of a plane mirror, its image is 8cm behind the mirror, so the candle and image are 16cm apart. The reflected ray that reaches your eye can be drawn as if it came in a straight line from that image.
Misconception check: the image behind a plane mirror is not a real object behind the glass. It is a virtual image because the reflected rays only appear to come from that position.
Refraction
When light enters denser medium (higher refractive index), it bends towards the normal; speed decreases.
Refraction direction checkpoint
For a refraction sketch, decide whether the ray slows down or speeds up before deciding which way it bends. Always measure the angle from the normal, not from the surface.
Ray path
Speed change
Angle to the normal
What to sketch
Common trap
Air into glass or water
Slower
Smaller
Bend the refracted ray towards the normal.
Drawing the ray closer to the surface.
Glass or water into air
Faster
Larger
Bend the refracted ray away from the normal.
Reversing the rule because the ray is leaving glass.
Along the normal
Speed changes, direction does not.
Stays at 0∘
Draw the ray straight through the boundary.
Forcing a bend even though there is no angle to change.
Through a rectangular glass block
Slower on entry, faster on exit.
Smaller on entry, larger on exit.
Draw the emergent ray parallel to the incident ray.
Making the final ray continue to bend further into the block.
Ray-sketch guide:
air | glass
incident \ | normal
\|
-----------+-----------
|\ refracted
| \
Worked check: a ray travelling from air into water at an angle to the normal slows down, so its angle to the normal becomes smaller. If the incident angle is 50∘, the refracted angle must be less than 50∘, not greater.
Misconception check: "towards the normal" means the angle between the ray and the normal gets smaller. It does not mean the ray moves towards the surface.
Snell's law for extension: n1sinθ1=n2sinθ2.
Dispersion
Prism splits white light into spectrum because different wavelengths refract at slightly different angles.
Worked example - Glass block experiment
Light enters glass at θ1=35∘, refractive index n=1.52.
sinθ2=1.52sin35∘=0.376⇒θ2=22.1∘.
Draw emergent ray parallel to incident ray when exiting a rectangular block (due to symmetrical refraction).
4 Sound wave interpretation
Oscilloscope trace amplitude corresponds to loudness; frequency determines pitch.
Doubling amplitude doubles energy carried, approximate 6 dB increase.
Higher frequency waves have shorter periods: T=f1.
Sound trace checkpoint
When comparing two oscilloscope traces, read the horizontal and vertical changes separately before describing the sound.
Trace change
What changes in the sound
What stays separate
Common trap
Taller wave on the screen
Louder sound because amplitude is larger.
Pitch is not decided by height.
Saying a taller trace has higher frequency.
More complete cycles in the same time interval
Higher pitch because frequency is larger.
Loudness is not decided by how close the waves are.
Saying a compressed trace is automatically louder.
Longer time for one complete cycle
Lower frequency, so lower pitch.
The time-base setting must be included if divisions are given.
Counting half a cycle as one full period.
Same shape but time-base setting changes
The displayed spacing changes even if the sound may not.
Always convert divisions into time before finding frequency.
Comparing screen spacing without checking the scale.
Worked check: Trace A is 3 divisions tall and takes 4 divisions for one cycle. Trace B is 1 division tall and takes 2 divisions for one cycle, with the same vertical scale and time-base setting. Trace A is louder because its amplitude is larger, but Trace B has the higher pitch because one cycle takes less time, so its frequency is higher.
Common trap: pitch and loudness are independent. A sound can be high-pitched and soft, or low-pitched and loud, depending on frequency and amplitude respectively.
Worked example - Frequency from oscilloscope
If 4 divisions represent one full cycle and time-base setting is 2.0ms⋅div−1:
T=4×2.0×10−3=8.0×10−3,s.
f=T1=125,Hz.
Try it yourself
Sketch particle displacement diagrams comparing transverse and longitudinal waves.
A light ray passes from air into water at 50∘ to the normal. Refractive index of water =1.33. Calculate refracted angle and sketch the ray.
Two tuning forks produce beats at 4Hz. One fork has frequency 256Hz. Determine possible frequencies of the second fork and describe how to identify the higher one experimentally.
