Night-before revision
Covers General Wave Properties I, Sound (Ch 10) and EM Waves (Ch 11). Work through it in this order:
- 10 min Read the formulas and exam traps. These are where most marks are lost.
- 10 min Flip through the definition cards and say each answer out loud before you flip.
- 15 min Do the drill. Redo only your wrong ones until you score 100%.
- 5 min Learn the EM spectrum order and one use each, then sleep. Sleep helps you remember more than cramming later.
1Formula sheet
f in Hz, T in s. Rearranged: T = 1/f
v in m/s, λ in m
same thing, because f = 1/T
Echo: d = vt ÷ 2 (the sound goes there and back)
direct sound, with no reflection
speed of all EM waves in a vacuum
| Prefix | Means | Example |
|---|---|---|
| m (milli) | × 10⁻³ | 0.5 ms = 5 × 10⁻⁴ s |
| c (centi) | × 10⁻² | 80 cm = 0.80 m |
| k (kilo) | × 10³ | 2 kHz = 2000 Hz |
| M (mega) | × 10⁶ | 1570 MHz = 1.57 × 10⁹ Hz |
| G (giga) | × 10⁹ | 5G uses the GHz range |
Solids > liquids > gases, because particles in solids are closely packed. Sound cannot travel through a vacuum.
2The 3 diagrams you must be able to label
3Definitions: exam wording
Tap a card to flip it. Say the definition first, then check. Use Hide all to test yourself.
4Exam traps: where marks are lost
The time given is for the trip there and back, so use d = vt/2. 4 s at 330 m/s is 660 m, not 1320 m.
Frequency stays the same (it is set by the source). Speed and wavelength both decrease going from a vacuum or air into glass or water.
If the x-axis is distance, you read λ. If it is time, you read T (then f = 1/T). Always read the axis label first.
Amplitude is measured from the rest position to the crest, so it is half the crest-to-trough height.
'3 waves in 1.0 m' gives λ = 1.0/3 = 0.333 m. 'Two waves in 0.04 s' gives T = 0.02 s. Divide by the number of waves.
Convert cm → m and ms → s before you substitute. Give answers to 3 s.f. with units.
Mobile phones and satellite communication use microwaves. Radio is used for broadcasting, long-distance communication (reflected off the ionosphere) and RFID.
Sound is longitudinal and needs a medium. EM waves are transverse and can travel through a vacuum at 3 × 10⁸ m/s.
Work out the path lengths first (direct, and via the wall). The time gap comes from the difference in distance: v = Δd / Δt.
Pitch ↔ frequency (more waves squeezed into the same time). Loudness ↔ amplitude (taller waves).
A particle at a crest or trough is momentarily at rest. For a wave moving right, a particle copies the particle just to its left.
Waves transfer energy. The particles only vibrate about a fixed position.
5EM spectrum in one look
R M I V U X G: Radio · Microwaves · Infrared · Visible · Ultraviolet · X-rays · Gamma
| Wave | Keyword to remember | Uses (any 2 = marks) |
|---|---|---|
| Radio | longest λ · ionosphere | radio/TV broadcasting, long-distance communication (3–30 MHz reflected off the ionosphere), police/military bands, RFID tags |
| Microwaves | phones · satellites | cooking, mobile phones, satellite communication, Wi-Fi, radar, GPS (≥ 3 satellites), cancer treatment |
| Infrared | heat · remote | heaters/ovens, remote controls, IR thermometers, intruder alarms, camera auto-focus, fibre-optic data, greenhouses |
| Visible | seeing · optical fibre | photography, endoscope/keyhole surgery, curing dental fillings, photographic film, plant-growth lights |
| Ultraviolet | sterilise · forgery | disinfection (water treatment, handrails), vitamin D, sun-tanning, anti-forgery on bank notes |
| X-rays | bones · CT | X-ray imaging (bone absorbs more, so it looks bright), CT scans, airport luggage scanning, finding cracks |
| Gamma | highest energy · cancer | radiotherapy (Gamma Knife), sterilisation, quality control (hidden cracks) |
Visible light: ROYGBIV = Red, Orange, Yellow, Green, Blue, Indigo, Violet (long λ → short λ).
6Answer templates: copy this layout
- Write v = 2d / t
- Substitute, e.g. 1200 = 2d / 0.8
- d = (1200 × 0.8) / 2
- d = 480 m ✓ units
- Distance axis → read λ; time axis → read T
- Convert to m and s
- f = 1/T, then v = fλ
- e.g. T = 0.02 s, λ = 3 m → v = 150 m/s
- Sketch the paths and add up each distance
- Two echoes: d₁ = v t₁ / 2, d₂ = v t₂ / 2
- Two bangs: v = (decho − ddirect) / Δt
- e.g. (650 − 350) / 1.0 = 300 m/s
- Air pumped out → sound becomes fainter
- Bell still seen ringing but no sound
- ∴ sound needs a medium; it cannot travel through a vacuum
- Find the original T (e.g. 2 s)
- 2× f → T halves (1 s): twice as many waves
- ½ f → T doubles (4 s)
- Keep the same amplitude unless told otherwise
- Highest pitch → most waves across the screen
- Loudest → tallest waves
- Check both conditions if the question asks for two