Waves

Explanation

• In a transverse wave, oscillations are perpendicular to the direction of energy transfer; in a longitudinal wave, oscillations are parallel to it.
• Use the direction the particles or points oscillate, not the direction the whole wave travels, to classify a wave.
• A floating marker can bob up and down while a ripple moves horizontally, showing that the disturbance and energy travel without the water moving along with the wave.
• Do not call every mechanical wave longitudinal: water-surface ripples are treated as transverse, while sound waves in air are longitudinal and contain compressions and rarefactions.

Explanation

• Amplitude is the maximum displacement from the undisturbed position; wavelength is the distance between equivalent points on adjacent waves; frequency is waves per second and period is time per wave.
• Measure several complete wavelengths or periods and divide by their number to reduce percentage uncertainty; for sound, two microphones and a measured separation can provide a travel time.
• Use $T=1/f$ and $v=f\lambda$.
• For example, a $5.0\,\text{Hz}$ wave has period $0.20\,\text{s}$.
• A common error is to use crest-to-trough distance as one wavelength; it is only half a wavelength, and amplitude must be measured from the undisturbed line rather than crest to trough.

Explanation

• At a boundary between materials, some incident wave energy may be reflected, some transmitted and some absorbed.
• For a reflection ray diagram, draw a normal perpendicular to the surface at the point of incidence and measure angles from the normal.
• The angle of incidence equals the angle of reflection.
• Energy accounting can test a boundary model: if $65\%$ is transmitted and $20\%$ reflected, the remaining $15\%$ is absorbed.
• Do not measure the angles from the surface or assume that every boundary reflects all of the incident wave.

Explanation

• Sound waves can make solids vibrate; in the ear, sound makes the eardrum and other structures vibrate, producing the sensation of sound.
• Trace a conversion by naming the incoming wave, the vibrating solid and any outgoing wave or electrical signal.
• Normal human hearing extends from about $20\,\text{Hz}$ to $20\,\text{kHz}$, so a $25\,\text{kHz}$ vibration is above the usual audible range.
• Do not assume that every vibrating system responds equally at all frequencies: conversion between sound and solid vibration works only over a limited frequency range.

Explanation

• Ultrasound is sound above $20\,\text{kHz}$; partial reflections at boundaries and their return times allow hidden interfaces to be located.
• For echo measurements use the total out-and-back distance, so a boundary distance is $d=vt/2$.
• P-waves are longitudinal and travel through solids and liquids, whereas transverse S-waves do not travel through liquids; their paths provide evidence about Earth's internal structure.
• A common error is to omit the factor of $2$ in echo sounding or to claim that an absent S-wave proves there are no waves rather than indicating a liquid region along its path.

Explanation

• Electromagnetic waves are transverse waves that transfer energy from a source to an absorber and form one continuous spectrum.
• Order the spectrum from long wavelength and low frequency to short wavelength and high frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
• All electromagnetic waves travel at the same speed in a vacuum, about $3.00\times10^8\,\text{m s}^{-1}$; for example, $f=5.0\times10^{10}\,\text{Hz}$ gives $\lambda=6.0\times10^{-3}\,\text{m}$.
• Do not say that higher-frequency electromagnetic waves travel faster in a vacuum; frequency and wavelength change across the spectrum, but the vacuum speed is the same.

Explanation

• Construct a refraction ray diagram using a normal at the boundary. Higher tier: AQA restricts "use wave front diagrams to explain refraction in terms of the change of speed that happens when a wave travels from one medium to a different medium" to Higher —
• tier, along with the statements that substances absorb, transmit, refract or reflect electromagnetic waves in ways that vary with wavelength and that refraction is due to the difference in wave velocity between substances.
• To compare infrared emission or absorption by surfaces, keep area, temperature, distance and detector geometry fixed, repeat readings and change only the surface finish.
• Higher only: on crossing into a slower medium, frequency stays constant, so wavelength decreases; closer wavefronts on the slower side show the speed change that causes refraction.
• Higher only: do not say refraction is caused by a frequency change; speed and wavelength change at the boundary, while frequency is fixed by the source.

Explanation

• Higher tier: AQA restricts "Radio waves can be produced by oscillations in electrical circuits" and the statement that absorbed radio waves "may create an alternating current with the same frequency as the radio wave itself" to Higher tier.
• Use dose data by converting units consistently: $1000\,\text{mSv}=1\,\text{Sv}$, then compare total doses rather than single exposures.
• Electromagnetic waves can be emitted or absorbed when atoms or nuclei change; gamma rays specifically originate from changes in an atomic nucleus.
• Do not treat ultraviolet, X-rays and gamma rays as equally hazardous: effects depend on radiation type and dose; ultraviolet can damage skin, while X-rays and gamma rays are ionising and can cause mutations and cancer.

Explanation

• Typical uses are radio for broadcasting; microwaves for satellite communication and cooking; infrared for heaters, cooking and thermal cameras; and visible light for fibre-optic communication.
• Higher tier: AQA restricts "give brief explanations why each type of electromagnetic wave is suitable for the practical application" to Higher tier; explain suitability by linking the application to whether the wave is transmitted, absorbed, detected or able to penetrate the relevant material.
• Ultraviolet is used in energy-efficient lamps and tanning, while X-rays and gamma rays are used for medical imaging and treatment.
• A common error is to name a use without explaining suitability; at Higher tier, link the wave's penetration, absorption or effect on matter to the application.

Explanation

• A convex lens refracts parallel rays towards its principal focus and may form real or virtual images; a concave lens spreads rays and always forms a virtual image.
• Construct a ray diagram with at least two standard rays from the same point on the object; their intersection, or the intersection of backward extensions, locates the image.
• Magnification is $\text{image height}/\text{object height}$ and has no unit; an image $4.5\,\text{cm}$ high from an object $1.5\,\text{cm}$ high has magnification $3.0$.
• Do not attach units to magnification or use mismatched units for the two heights; a virtual image cannot be projected onto a screen.

Explanation

• Each visible colour occupies a narrow range of wavelengths and frequencies within the electromagnetic spectrum.
• A smooth surface gives specular reflection mainly in one direction; a rough surface gives diffuse reflection by scattering light, while transparent and translucent materials both transmit light but differ in image clarity.
• A colour filter absorbs some wavelength ranges and transmits others, so predict the emerging light by finding the wavelengths that both arrive and pass through the filter.
• An opaque object appears the colour it reflects most strongly; it appears white if it reflects all visible wavelengths similarly and black if it absorbs them all.
• Do not say an ordinary coloured object produces its own light.

Explanation

• Every object emits and absorbs infrared radiation, and a hotter object emits more infrared energy in a given time.
• To compare surfaces fairly, use equal areas at the same temperature and keep detector distance, angle and surroundings constant; repeat readings before comparing means.
• A perfect black body absorbs all incident radiation, reflecting and transmitting none; because a good absorber is also a good emitter, it is the best possible emitter.
• Do not confuse visible colour alone with the experimental variable: surface finish matters, and a shiny surface is generally a poorer absorber and emitter than a matt black surface.

Explanation

• All objects emit radiation, and both the intensity and wavelength distribution of the emitted radiation depend on temperature.
• Higher tier: AQA restricts "explain how the temperature of a body is related to the balance between incoming radiation absorbed and radiation emitted, using everyday examples to illustrate this balance,
• and the example of the factors which determine the temperature of the Earth" to Higher tier, together with drawing or interpreting diagrams of how radiation affects the temperature of Earth's surface and atmosphere;
• equal rates of absorption and emission mean constant temperature. Higher only: if a body absorbs $480\,\text{J}$ each second but emits $530\,\text{J}$ each second, it has a net energy loss of $50\,\text{J s}^{-1}$ and cools.
• Higher only: do not infer constant temperature from a constant incoming rate alone; reflection, absorption and emission all affect the balance, including for Earth's surface and atmosphere.