Waves

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.

Longitudinal waves show compressions 密部 (particles squashed) and rarefactions 疏部 (particles spread).

Evidence that the wave travels, not the material: a ripple moves across a pond but the water itself just bobs up and down (a ball on the surface stays put); sound reaches you but the air does not travel from source to ear.

• Wave speed is the speed at which energy is transferred through the medium.
• Read amplitude and wavelength straight off a labelled diagram.

Worked example. A water wave has frequency 2.0 Hz and wavelength 0.35 m.

Worked example (kHz and μm). Sound of frequency 4.0 kHz travels at 330 m/s.

• Convert: $f = 4000$ Hz.
  $$\lambda = \frac{v}{f} = \frac{330}{4000} = 0.0825 \approx 8.3\times10^{-2}\ \text{m}$$

Measuring wave speeds (RP8)

• Ripples: darkened ripple tank, straight-bar motor makes continuous waves; photograph/measure the wavelength with a ruler on the screen, count waves passing a point in 10 s for frequency; $v = f\lambda$.
• Waves in a solid: a vibration generator sends waves along a stretched string; adjust the frequency until a clear whole number of loops appears — measure the length and count loops for $\lambda$; $f$ is read from the signal generator.
• Speed of sound: stand a known distance from a wall, clap and time the echo for many claps, divide (or use two people with a stopwatch over a large distance; electronic timing is better).

(Physics only) Sound changing medium: if speed changes, either frequency or wavelength (or both) change with it — $v = f\lambda$ links all three.

At a boundary a wave may be reflected, absorbed or transmitted:

• specular reflection 镜面反射: from a smooth surface, one direction;
• diffuse reflection 漫反射: from a rough surface, scattered;
• absorption: energy stays in the material; transmission: passes through.

Construct the reflection ray diagram: the normal at right angles to the surface at the point of incidence; the angle of incidence equals the angle of reflection — both measured from the normal.

RP9: shine a ray box at plane mirror / rough surfaces; trace incident and reflected rays with a pencil, measure angles with a protractor; for refraction, pass light through a glass block and trace the bent path at each boundary.

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).

A lens forms an image by refracting light:

• convex 凸透镜: parallel rays converge at the principal focus; focal length = lens-to-focus distance; images real or virtual.
• concave 凹透镜: rays spread; image always virtual.

Ray-diagram rules (two rays locate the image): a ray parallel to the axis refracts through the focus (convex) or appears to come from it (concave); a ray through the centre of the lens goes straight on.

• A ratio, no units; both heights in mm or both in cm.

Worked example. An object 5.0 mm high forms an image 20 mm high.

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).