AQA GCSE Physics (8463) · Paper 2
🌊 Waves
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Wave basics
Waves carry energy and information, but not matter. A cork floating on ripples just bobs up and down, while the water itself stays where it is. Transverse waves oscillate at right angles to the direction of travel, as in light and ripples on water, while longitudinal waves oscillate along the direction of travel, as compressions and rarefactions; sound is an example.
Amplitude is the maximum displacement of a wave from its rest position. Wavelength is the length of one full cycle of the wave. Frequency is the number of waves passing a point per second, measured in hertz. The speed of sound can be measured with the echo method: stand a measured distance from a wall, clap, and time the echoes; the speed is then 2 × distance ÷ time, because the sound travels to the wall and back again.
The EM spectrum
The electromagnetic spectrum is one continuous family of waves that differ only in wavelength: in order, radio, microwave, infrared, visible, ultraviolet (UV), X-ray and gamma waves, with frequency rising and wavelength falling along that order. All of these waves are transverse, and all travel at the same speed in a vacuum.
Higher content: radio waves can be produced by oscillations in electrical circuits; when they are absorbed by a receiving aerial they induce a matching alternating current in it.
Refraction happens when a wave crosses a boundary and changes speed, which bends its direction unless the wave hits the boundary straight on.
Higher content: one edge of the wavefront reaches the boundary and changes speed before the rest of the wavefront does, which turns the wave's direction as it crosses.
Sound, seismic & lenses (Triple) Separate science only
Sound is a vibration, and human hearing ranges from about 20 Hz to 20 kHz. Ultrasound is sound above 20 kHz; it partially reflects at boundaries between different materials, which is the basis of baby scans, flaw detection in metal, and sonar.
Seismic waves are used to study the structure of the Earth. P-waves are longitudinal and pass through both solids and liquids, while S-waves are transverse and pass through solids only. Because S-waves cannot pass through the core, this is evidence that the outer core is liquid.
Light
The law of reflection states that the angle of incidence equals the angle of reflection, both measured from the normal, an imaginary line at 90° to the surface. Specular reflection happens at a smooth surface and produces a clear mirror image, while diffuse reflection happens at a rough surface and scatters light in all directions, producing no image.
A convex lens converges light rays to a principal focus; the distance from the lens to this point is called the focal length. A convex lens can form a real image, which can be projected on a screen, or, when the object is close to the lens, a magnified virtual image. A concave lens spreads light rays apart and always produces a smaller virtual image. Magnification = image height ÷ object height.
The colour of an object depends on which wavelengths its surface absorbs and which it reflects; a red bus looks red because it reflects red light and absorbs the other colours. Filters transmit only their own colour of light. A white surface reflects all colours, and a black surface absorbs all colours. A transparent material can be seen through clearly, a translucent material lets light through but scatters it, and an opaque material does not let light through at all.
A perfect black body absorbs all of the radiation hitting it, and it is also the best possible emitter. Hotter objects radiate more energy, and the peak of their radiation shifts to shorter wavelengths as they heat up, from red-hot to white-hot. The Earth's temperature settles at the point where the energy it absorbs equals the energy it radiates back into space.