Your revision route
What you’ll learn
- Distinguish transverse and longitudinal waves and explain evidence that the medium is not carried with the wave.
- Define amplitude, wavelength, frequency and period, and use T = 1/f and v = fλ.
- Describe methods for measuring sound speed and ripple speed, frequency and wavelength.
- Draw refraction ray diagrams; Separate Physics: explain reflection, transmission and absorption and draw reflection rays.
- Order the electromagnetic spectrum and compare its uses, production and hazards.
- Higher Tier: explain wavelength-dependent behaviour, radio-wave circuits and why electromagnetic waves suit particular uses.
- Separate Physics: explain sound crossing media, lenses, colour and infrared; Separate Physics Higher: explain hearing, ultrasound and seismic exploration.
- Plan, analyse and improve the ripple-tank and infrared practicals; Separate Physics: do the same for the light practical.
Build the big picture
Key ideas
The disturbance travels; the medium mostly stays
Drop a cork into ripples and it bobs rather than surfing to shore: energy travels while matter oscillates locally.
- Transverse oscillations are perpendicular to energy transfer; longitudinal oscillations are parallel.
- Water-surface ripples are treated as transverse here; sound in air is longitudinal, with compressions and rarefactions.
- Evidence that water or air is not carried along is the local back-and-forth motion around an equilibrium position.
The punchline: Compare the two directions: oscillation and energy transfer.
A wave has a height, a spacing and a rhythm
Amplitude records the furthest excursion from equilibrium, wavelength the spacing of one full cycle and frequency the number of cycles each second.
- Measure amplitude from equilibrium to the furthest crest or trough, not across the full crest-to-trough height.
- Measure one wavelength along the direction of travel between successive in-phase landmarks, such as crests or compressions.
- Frequency is measured in hertz; period is time per cycle, so T = 1/f.
- Wave speed is the speed of energy transfer and obeys v = fλ.
The punchline: Mark the equilibrium line before identifying amplitude or wavelength.
Measure distance and time more than one tiny event
A longer measured distance or several cycles reduces percentage uncertainty and makes a wave-speed result less fragile.
- For sound, measure a known separation and a travel-time delay, or use two microphones and an oscilloscope; calculate distance ÷ time.
- For ripples, count waves per second for frequency and measure several wavelengths, then divide by the number of waves.
- Use v = fλ, repeat measurements and report a mean while keeping water depth and source conditions controlled.
- The waves-in-a-solid activity also tests whether the apparatus can resolve frequency, wavelength and speed reliably.
The punchline: Measure a large interval, divide carefully, repeat and state the control variables.
Read a waveform without guessing
The equilibrium line is the ruler's zero. Measure amplitude vertically from it, then measure one horizontal cycle between matching landmarks.
A boundary asks different questions by route
Separate Physics: reflection returns a wave into the original medium, transmission carries it across the boundary, and absorption transfers wave energy to the material.
- Separate Physics: draw reflected rays with equal angles measured from the normal, not the surface.
- All routes: construct refraction ray diagrams at a boundary between two media.
- Separate Physics: for sound crossing a medium boundary, frequency stays fixed while speed and wavelength change together.
- Higher Tier, both routes: a slower medium bends the ray towards the normal; a faster medium bends it away. Along the normal, speed changes but direction does not.
The punchline: Separate Physics, both tiers: sound frequency stays fixed. Higher Tier, both routes: an EM wave front turns as the side entering first changes speed first.
Separate Physics Higher: echoes reveal hidden boundaries
Reflections, absorption and velocity differences let waves expose structures hidden from direct observation; seismic evidence has revealed parts of Earth we cannot observe directly.
- Sound waves vibrate solids; in the ear they vibrate the eardrum and linked parts, producing the sensation of sound. Normal human hearing spans about 20 Hz to 20 kHz.
- The ear's solid parts have mass, stiffness and damping, so response depends on frequency. Beyond the effective range, too little vibration passes onwards to produce a sensation of sound.
- Ultrasound is above 20 kHz, partly reflected and weakened by absorption. Timed echoes locate medical/industrial boundaries; high-frequency echo sounding finds deep-water objects and depth.
- Earthquakes produce seismic waves. Longitudinal P-waves have different speeds in solids and liquids; transverse S-waves cannot cross liquid. Together they reveal core structure and size.
The punchline: For an echo, distance = wave speed × round-trip time ÷ 2. Absorption weakens what returns; reflection marks the boundary.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Turn the wave dials
Explore wave speed, frequency and wavelength
Choose frequency and wavelength values to explore how their product gives wave speed.
v = f × λ
Wave speed40 m/s
The sliders vary f and λ independently only to explore the equation. In one fixed, non-dispersive medium, speed is set by the medium, so changing f changes λ rather than speed.
Walk the spectrum
Order the electromagnetic spectrum
Arrange the regions from longest wavelength and lowest frequency to shortest wavelength and highest frequency.
Choose the first step below.
The sequence is continuous, not seven separate boxes. Moving along it, wavelength falls while frequency rises; speed in a vacuum or air stays the same.
Trace the rays
Separate Physics: predict the image
Choose a complete lens-and-object scenario. Read the ray behaviour before naming the image.
For a convex lens with the object beyond the focal length, refracted rays converge on the far side, so the image can be projected onto a screen.
1 of 3 states explored
A convex lens can form real or virtual images. A concave lens makes rays diverge and produces an upright, diminished virtual image for a real object.
The electromagnetic spectrum is one continuous family
Radio waves and gamma rays differ enormously in scale, yet both are transverse and travel at the same speed in a vacuum or air.
- From long wavelength and low frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- All transfer energy from a source to an absorber; human eyes detect only visible light.
- Changes in atoms can generate or absorb electromagnetic waves; gamma rays originate in nuclear changes.
- Higher Tier: oscillating circuits produce radio waves, and absorbed radio waves can induce alternating current at the same frequency.
The punchline: As wavelength falls across the spectrum, frequency rises.
A useful wave is one that interacts in the right way
Higher Tier, both routes: wavelength changes a material's response. It may absorb at one wavelength but transmit, refract or reflect at another, helping choose a wave for a task.
- All routes: radio serves TV and radio; microwaves serve satellites and cooking. Higher Tier links aerial response, transmission or absorption to the use.
- All routes: visible light carries signals through fibres; infrared serves heating and cameras. Higher Tier: repeated reflection confines pulses; absorption heats; emission reveals.
- All routes: ultraviolet serves lamps and tanning; exposure can age skin and raise cancer risk. Higher Tier explains fluorescent-coating or skin absorption.
- All routes: X-rays and gamma serve imaging and treatment. Higher Tier: absorbed X-rays reveal bone or ionise cells; tracer gamma reaches detectors; targeted gamma ionises deep tumour cells.
The punchline: Higher Tier, both routes: application answers need a property → interaction → purpose chain.
Separate Physics: a lens redirects a bundle of rays
A convex lens gathers parallel rays towards a principal focus; a concave lens spreads them as if from a focus.
- Focal length is the separation between a lens and its principal focus.
- A convex lens can form a real image, where rays meet, or a virtual image, where backward extensions meet.
- A concave lens forms a virtual image for a real object.
- Standard symbols: a convex lens is a line with outward-pointing arrowheads; a concave lens has inward-pointing arrowheads. Magnification = image height ÷ object height; no unit.
The punchline: Construct the rays first; let their meeting point decide the image.
Watch refraction turn
Angles are measured from the normal. At Higher Tier, wave fronts reveal the cause: one side changes speed first, so an oblique wave turns.
One family, many jobs
All routes learn the applications below. Higher Tier links each use to transmission, reflection, absorption, penetration or induced current.
- Radio and microwaveRadio induces matching-frequency currents in aerials; microwaves cross the atmosphere for satellites or are absorbed by food for heating.
- Infrared and visibleInfrared is absorbed for heating and emitted for cameras to detect; repeated reflection keeps coded visible-light pulses bouncing through thin fibres with little signal loss.
- UltravioletAbsorption makes fluorescent lamp coatings emit visible light and affects skin in tanning; exposure can age skin and increase cancer risk.
- X-rays and gammaX-rays penetrate soft tissue but bone absorbs more, creating contrast; gamma from tracers reaches a detector. Absorbed X-rays or gamma ionise and destroy tumour cells.
Let the rays decide the image
Separate Physics uses standard lens symbols and two dependable construction rays. Solid rays show real paths; dashed extensions locate a virtual image.
Separate Physics: colour is selective survival
Each visible colour occupies a narrow wavelength-and-frequency band; an object's colour depends on which bands reach the eye.
- Smooth surfaces give specular reflection in one direction; rough surfaces scatter light by diffuse reflection.
- A filter transmits selected wavelengths and absorbs others.
- An opaque object reflects some wavelengths more strongly; white reflects all roughly equally and black absorbs all.
- Transparent materials transmit light clearly; translucent materials transmit while scattering it.
The punchline: Track the incident wavelengths, then remove those absorbed by the filter or object.
Separate Physics: black bodies exchange radiation
Every object emits and absorbs infrared; temperature changes both emission intensity and wavelength distribution.
- A hotter object emits more infrared radiation in a given time.
- A perfect black body takes in every incoming wavelength, reflecting and transmitting none; at a fixed temperature it is the strongest possible thermal emitter.
- Separate Physics Higher: equal absorbed and emitted rates keep temperature constant; a hot drink cools when emission wins and a sunlit dark surface warms when absorption wins.
- Separate Physics Higher: Earth's temperature depends on absorption, emission and reflection involving the surface, atmosphere and space.
The punchline: Temperature follows the balance of incoming absorbed and outgoing emitted radiation.
Words worth knowing
Key definitions
- amplitude
- The greatest distance reached from the equilibrium position during an oscillation.
- wavelength
- The spacing, along the direction of travel, between successive points in the same phase, such as neighbouring crests or compressions.
- frequency
- The number of complete oscillations or cycles per second, measured in hertz (Hz). For a travelling wave, it is also the number of complete cycles passing a point each second.
- period
- The time taken for one complete wave or oscillation, measured in seconds.
- refraction
- A direction change that can occur when a wave crosses into another medium. Higher Tier: a speed change bends a wave entering obliquely; at normal incidence, speed and wavelength change but direction does not.
- Separate Physics Higher: ultrasound
- Sound above about 20 kHz—too high in frequency for people to hear.
- Separate Physics: real image
- An image formed where light rays actually converge, so it can be projected onto a screen.
- Separate Physics: virtual image
- An image formed where diverging rays appear to originate; it cannot be projected onto a screen.
- Separate Physics: perfect black body
- An ideal object that takes in every wavelength reaching it, reflects or transmits none, and emits the maximum thermal radiation possible at its temperature.
- radiation dose
- Radiation dose, recorded in sieverts (Sv), helps quantify the chance of harm after exposure; 1000 mSv = 1 Sv and the unit need not be recalled. Risk also depends on dose, radiation type and exposure conditions.
Calculate with confidence
Equations
Period and frequency
T = 1 / f
The period is the reciprocal of frequency.
| Symbol | Meaning | Unit |
|---|---|---|
| T | period | s |
| f | frequency | Hz |
Exam tip: You receive this equation on the Physics sheet; check that frequency is in hertz.
Wave speed
v = f × λ
Wave speed equals frequency multiplied by wavelength.
| Symbol | Meaning | Unit |
|---|---|---|
| v | wave speed | m/s |
| f | frequency | Hz |
| λ | wavelength | m |
Exam tip: Recall this equation and convert centimetres or millimetres to metres before substituting.
Separate Physics: magnification
magnification = image height / object height
Magnification compares image size with object size and has no unit.
| Symbol | Meaning | Unit |
|---|---|---|
| magnification | image-to-object size ratio | no unit |
| image height | height of image | mm or cm |
| object height | height of object | same unit as image height |
Exam tip: You receive this equation on the Physics sheet. Use the same length unit for image and object so the units cancel.
Follow it step by step
Processes to remember
How to draw a refraction ray diagram
- Draw the boundary and a dashed normal at 90° where the ray arrives.
- Draw the incident ray with an arrow towards the boundary.
- All routes: construct the refracted ray with an arrow away from the boundary, using the stated media and diagram information to determine its direction.
- Higher Tier, both routes: a slower medium bends the ray towards the normal; a faster medium bends it away; incidence along the normal produces no bend. Frequency stays fixed while wavelength changes.
- Measure incidence and refraction angles from the normal.
Exam tip: Higher Tier, both routes: a ray along the normal can change speed and wavelength without changing direction.
Higher Tier, both routes: explain an electromagnetic-wave use
- Name the spectrum region used.
- State the relevant property, such as penetration, absorption, reflection or transmission.
- Explain how that interaction performs the stated task.
- If asked to evaluate, compare benefit with hazard using the dose or outcome data supplied.
Exam tip: Higher Tier, both routes: build a property-to-purpose link, not a memorised list.
Separate Physics: construct convex- and concave-lens ray diagrams
- Draw the principal axis, lens, a principal focus on each side and the object.
- Draw a ray from the object top parallel to the axis: through the far focus after a convex lens, or diverging as if from the near focus after a concave lens.
- Draw a second ray through the optical centre without changing direction.
- For a convex lens, draw the image where actual rays meet; if they diverge, extend them backwards with dashed lines to locate a virtual image.
- For a concave lens, extend the diverging rays backwards with dashed lines until they meet; draw the upright virtual image there.
Exam tip: Use a ruler and arrowheads, and identify real or virtual from whether the actual rays meet.
See the thinking
Worked example
Worked example: read one wave two ways
Water ripples have frequency 5.0 Hz and wavelength 0.24 m. Calculate their speed and period.
- Use v = f × λ.
- Substitute: v = 5.0 × 0.24 = 1.2 m/s.
- Use T = 1/f.
- Calculate: T = 1/5.0 = 0.20 s.
Answer: The wave speed is 1.2 m/s and the period is 0.20 s.
Frequency counts five waves each second, so one cycle lasts one fifth of a second. Multiplying that frequency by the wavelength gives the distance the wave pattern advances each second.
Protect the marks
Common mistakes
Watch out: Measuring amplitude from trough to crest.
Do this instead: Read amplitude as the equilibrium-to-furthest-point distance; the full trough-to-crest height is twice that value.
Watch out: Saying particles travel with the wave.
Do this instead: Particles oscillate locally while the disturbance and energy travel through the medium.
Watch out: Changing frequency when sound crosses from one medium to another.
Do this instead: Separate Physics, both tiers: the source fixes sound frequency; speed and wavelength change together.
Watch out: Measuring a ray angle from the surface.
Do this instead: All routes: measure incidence and refraction angles from the normal. Separate Physics: reflection angles also use the normal.
Watch out: Writing that all electromagnetic waves have different speeds in a vacuum.
Do this instead: All electromagnetic waves travel at the same speed in a vacuum or air.
Watch out: Separate Physics: calling every convex-lens image real.
Do this instead: Separate Physics: a convex lens forms a virtual image when the object is inside the focal length.
Plan it like the exam
Required practicals
Measure frequency, wavelength and wave speed
Combined Science and separate Physics
Aim: Measure frequency, wavelength and wave speed for water ripples and transverse waves on a stretched string.
Method
- Set a ripple tank on a stable bench with shallow water and a low-voltage vibration source; view wave fronts against a scale.
- Watch one position, count several crests arriving there and time the interval; then calculate frequency = count ÷ time.
- Measure the distance across several wavelengths and divide by the number of wavelengths.
- Calculate v = fλ and repeat at fixed water depth and source setting.
- For a solid, attach a stretched string to a low-voltage vibration generator; pass it over a secured pulley to a known hanging mass that sets the tension.
- Set a low amplitude and adjust the signal-generator frequency until the string shows a clear stationary wave; record the frequency.
- Measure the vibrating length L across n half-wavelength loops, calculate λ = 2L/n, then calculate v = fλ.
- Repeat at other resonant frequencies while keeping the same string, vibrating length, hanging mass and source amplitude; compare a mean speed.
Variables
- Independent
- source frequency or the water/string wave system being measured
- Dependent
- measured wavelength and calculated wave speed
- Controls
- water depth
- source amplitude
- measurement region
- apparatus alignment
- string material and linear density
- hanging mass and string tension
- vibrating string length
Analysis: For ripples, use several cycles and wavefront gaps. For the string, use λ = 2L/n from several loops. Calculate v = fλ, repeat and compare mean speeds with uncertainty.
Safety
- Use only the teacher-approved low-voltage supply and switch it off before adjusting the vibrator.
- Keep water away from electrical connections and wipe spills immediately.
- Keep the tank stable and follow the school's instructions for the solid-wave apparatus.
- Secure the pulley and clamps, wear eye protection, do not overstretch the string and keep feet clear of hanging masses.
Improvements
- Use a strobe or video freeze-frame to make moving wave fronts easier to measure.
- Measure several wavelengths and time several cycles rather than one.
- Repeat each condition and use a mean after checking anomalies.
Investigate reflection and refraction of light
Separate Physics only
Aim: Separately compare reflected brightness, sharpness and scatter from surfaces, then reconstruct refraction through transparent blocks.
Method
- Surface test: set a ray box at a fixed distance and incident angle from the first surface; keep light colour, beam width and source setting unchanged.
- Measure reflected brightness with a light sensor at a fixed distance and angle; record reflected-patch sharpness or width and the spread of scattered light.
- Repeat for smooth, matt and rough surfaces, resetting the same geometry and taking repeat readings.
- Block test: place a transparent rectangular block on plain paper, draw its outline and send one narrow ray into it at a chosen angle.
- Mark two points on the incident ray and two on the emergent ray, then switch off the ray box and remove the block.
- Join the marks with a ruler and reconstruct the ray through the block; draw normals and measure incidence and refraction angles from them.
- Repeat for several incident angles and with at least one different transparent substance, changing only the intended angle or material.
Variables
- Independent
- surface finish, or incident angle/transparent material for the block test
- Dependent
- reflected brightness, sharpness and scatter, or reconstructed ray angles
- Controls
- light colour
- ray width
- source setting and distance
- sensor distance and angle
- component position and outline
- angle-measurement method
Analysis: Compare mean reflected brightness, patch sharpness or width and scatter only at matched geometry. For each block, reconstruct the internal ray and compare angles measured from the normals.
Safety
- Never look directly into the ray box or aim it at anyone's eyes.
- Switch off and allow a hot lamp housing to cool before moving it.
- Handle glass blocks carefully and report chips or breakage to the teacher.
Improvements
- Mark widely separated points on each ray to reduce angular uncertainty.
- Use a narrow beam and a sharp pencil line.
- Repeat each angle and use a mean after checking the normal and component outline.
Investigate infrared absorption and emission by surfaces
Combined Science and separate Physics
Aim: Compare how surface nature affects infrared absorption or emission under controlled conditions.
Method
- For emission, fill a Leslie cube with hot water as directed and measure infrared intensity from each surface at the same distance.
- For absorption, place identical sensors or containers with different surfaces at equal distances from an infrared source.
- Record starting readings, expose each surface for the same time and record temperature or detector change.
- Repeat readings while keeping geometry, starting temperature and exposure time constant.
- Compare colours while finish is fixed, then finishes while colour is fixed; repeat each matched condition.
Variables
- Independent
- surface colour with finish fixed, or surface finish with colour fixed
- Dependent
- infrared detector reading or temperature change over a fixed time
- Controls
- distance and angle
- surface area
- starting temperature
- exposure time
- room conditions
Analysis: Compare mean readings within matched sets. Black surfaces and matt surfaces usually absorb and emit more infrared than light or shiny surfaces; keep each conclusion tied to the apparatus used.
Safety
- Use hot rather than boiling water and keep the cube on a stable heatproof mat.
- Do not touch a hot lamp, cube or surface; allow equipment to cool before moving it.
- Keep liquids away from electrical equipment and follow the school's risk assessment.
Improvements
- Clamp the detector so distance and angle stay fixed.
- Use temperature probes or data logging for consistent timing.
- Randomise or repeat the surface order to reduce drift as the source cools.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
How do transverse and longitudinal waves differ?
Answer: Their oscillations are perpendicular and parallel to energy transfer, respectively.
Compare vibration direction with the direction in which the wave transfers energy.
A crest is 3 cm above equilibrium and a trough 3 cm below. What is the amplitude?
Answer: 3 cm
The equilibrium-to-extreme distance is 3 cm, half the 6 cm crest-to-trough span.
A 12 Hz wave has wavelength 0.50 m. What is its speed?
Answer: 6.0 m/s
v = fλ = 12 × 0.50 = 6.0 m/s.
Higher Tier, both routes: which quantity stays fixed when an electromagnetic wave refracts into another medium?
Answer: Frequency
The source fixes the electromagnetic wave's frequency; changing speed changes wavelength and, for oblique incidence, direction.
Which region lies between infrared and ultraviolet in the electromagnetic spectrum?
Answer: Visible light
The order is microwave → infrared → visible → ultraviolet → X-ray.
Separate Physics: what makes a perfect black body special?
Answer: It takes in every arriving wavelength, returns none by reflection or transmission, and emits the maximum thermal radiation possible at its temperature.
Surfaces that take in radiation efficiently also radiate efficiently; the perfect black body is the theoretical limit.
Good questions, clear answers
Frequently asked questions
Do waves carry matter from source to detector?
Waves transfer energy. In a material wave, particles usually oscillate around equilibrium positions rather than travelling the full distance with the disturbance.
Which medium-boundary changes are Separate Physics, and which are Higher Tier?
Separate Physics, both tiers: for sound, frequency stays fixed while speed and wavelength change. Higher Tier, both routes: for electromagnetic refraction, frequency stays fixed while speed, wavelength and sometimes direction change.
Are all electromagnetic waves dangerous?
They can all transfer energy, but harm depends on radiation type and dose. Ultraviolet can damage skin; ionising X-rays and gamma can mutate genes and cause cancer.
Separate Physics Higher: why is an ultrasound echo useful?
Ultrasound partly reflects where materials meet. Measuring the return time, with the wave speed and two-way journey, can locate a hidden boundary. This detail is Separate Physics Higher Tier.
Which waves content is Separate Physics only?
Sound-medium changes, reflection, light practical work, lenses, colour and infrared/black-body theory are Separate Physics. Hearing, ultrasound and seismic detection, and radiation balance are Separate Physics Higher.
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