波

Ripples on a pond, the sound of a voice, the light of a distant star — all are waves carrying energy from a source to an absorber. This reference covers AQA GCSE Physics 8463, topic 4.6 Waves.

How the exam treats this topic:

• Paper 2 carries this topic. $T = 1/f$, $v = f\lambda$ and magnification are on the enclosed sheet.
• Reflection (RP9), sound, detection waves, lenses, visible light and black-body radiation are physics only; sound and detection are also HT only; parts of EM properties are HT only.
• Required practicals: RP8 (wave speed in a ripple tank and a solid) and RP9 (reflection and refraction, physics only).
• You must construct ray diagrams for reflection, refraction and lenses.

Sound travels through solids as vibrations. In the ear, sound waves vibrate the ear drum and other parts — the sensation of sound is vibration converted. This works only over a limited frequency range: human hearing spans 20 Hz to 20 kHz. Examples of conversion: a microphone's diaphragm, a drum skin, windows rattling near a bass speaker.

• Ultrasound: frequency above 20 kHz; partially reflected at boundaries between media; the echo time gives the distance to a boundary ($s = vt$, with the path often there-and-back). Uses: medical prenatal scanning (safe, non-ionising), industrial flaw detection.
• Seismic waves: earthquakes produce P-waves (longitudinal) and S-waves (transverse), travelling at different speeds through the Earth; P-waves pass through liquids, S-waves do not — the shadow zones reveal the Earth's layered structure. Echo sounding with ultrasound/sound pulses maps seabeds.

All EM waves are transverse, transferring energy from source to absorber. They form a continuous spectrum and all travel at the same speed in vacuum or air ($3\times10^8$ m/s). From long to short wavelength:

Eyes detect only visible light — a tiny band.

Hazards: UV ages skin prematurely and raises skin-cancer risk; X-rays and gamma rays are ionising — they can mutate genes and cause cancer. Radiation dose in sieverts measures the risk of harm (1000 mSv = 1 Sv; recall of the unit not required). Draw conclusions from dose data.

(HT) Substances absorb, transmit, refract or reflect EM waves in ways that vary with wavelength; refraction comes from the change of speed between substances. Show refraction on a ray diagram (bending towards the normal when slowing) and on wavefront diagrams (wavefronts closer together in the slower medium).

Each colour is its own narrow band of wavelength (red longest, violet shortest in the visible band).

• Filters absorb some wavelengths and transmit others (a red filter transmits red).
• An opaque object's colour = the wavelengths it strongly reflects; the rest are absorbed. All reflected → white; all absorbed → black.
• Transparent/translucent objects transmit light.
• Specular vs diffuse reflection (from the reflection section) explains why a smooth red surface looks glossy but paper looks matt.

All bodies, at any temperature, emit and absorb infrared. The hotter the body, the more radiation it emits per second.

A perfect black body absorbs all incident radiation — no reflection, no transmission — and (good absorber = good emitter) is also the best possible emitter.

The intensity and wavelength distribution of the emitted radiation depend on the body's temperature: hotter → more intense, and the peak shifts to shorter wavelength.

(HT) A body at constant temperature absorbs at the same rate as it emits.

Absorbing faster than emitting → temperature rises. The Earth's temperature depends on the balance of absorbed and emitted radiation and on reflection back to space — use it to explain warming and ice-albedo style examples, and read the standard diagram.

• Define amplitude, wavelength, frequency, period; use $T = 1/f$ and $v = f\lambda$ with prefixes.
• Describe RP8 in a ripple tank and on a string; describe a speed-of-sound method.
• (physics only) Draw reflection and refraction ray diagrams with the normal; RP9.
• Recite the EM spectrum order; match uses and hazards with reasons; compare dose data.
• (physics only) Draw lens ray diagrams (convex/concave); magnification as a unitless ratio.
• (physics only) Explain colour by reflection, filters by transmission.
• (physics only) Black-body emission, absorption and the Earth's radiation balance (HT).