Glossary — Waves
Absorption — the taking in of electromagnetic radiation by a material. The energy of the radiation is transferred to the thermal store of the material.
Amplitude — the maximum displacement of a point on a wave from its undisturbed position.
Angle of incidence — the angle between an incident ray and the normal.
Angle of reflection — the angle between a reflected ray and the normal. It equals the angle of incidence.
Black body radiation — the electromagnetic radiation emitted by an object because of its temperature. A hotter object emits more radiation, with its most intense wavelength at a shorter wavelength.
Compression — a region of a longitudinal wave where the particles are closer together than in the undisturbed medium.
Concave lens — a lens that is thinner in the middle than at the edges. It spreads parallel rays of light apart, so it is a diverging lens.
Convex lens — a lens that is thicker in the middle than at the edges. It brings parallel rays of light together, so it is a converging lens.
Dispersion — the splitting of white light into the colours of the spectrum, because each colour is refracted by a different amount.
Echo — a reflected sound wave, heard separately from the original sound.
Echo sounding — a method that uses high-frequency sound to detect objects in deep water and to measure the depth of the water.
Electromagnetic spectrum — the seven groups of electromagnetic waves, from radio waves at the longest wavelength to gamma rays at the shortest.
Electromagnetic wave — a transverse wave made of a vibrating electric field and a vibrating magnetic field. It carries energy and does not need a medium.
Filter — a piece of transparent material that transmits some wavelengths of light and absorbs the rest.
Focal length — the distance from the centre of a lens to its principal focus.
Frequency — the number of waves that pass a point each second, measured in hertz.
Hertz — the unit of frequency; one hertz is one wave per second.
Infrared — electromagnetic radiation with a wavelength a little longer than red light. Warm objects emit infrared radiation, and it is felt as heat.
Infrasound — sound with a frequency below the range of normal human hearing, so below about 20 Hz.
Ionising radiation — radiation with enough energy to remove electrons from atoms. Ultraviolet, X-rays and gamma rays are ionising.
Longitudinal wave — a wave in which the vibration is along the same line as the direction in which the wave travels. A sound wave in air is longitudinal.
Magnification — the image height divided by the object height. It has no unit.
Material interface — the boundary between two different materials.
Medium — the material that a wave passes through.
Normal — a line drawn at right angles to a surface at the point where a ray meets it. Angles of incidence and reflection are measured from the normal.
P-wave — a longitudinal seismic wave produced by an earthquake. P-waves travel through both solids and liquids.
Period — the time for one complete wave to pass a point, measured in seconds. The period is the reciprocal of the frequency.
Principal focus — the point where a convex lens brings rays that are parallel to the principal axis together.
Pulse — a single short disturbance that travels as a wave.
Rarefaction — a region of a longitudinal wave where the particles are spread further apart than in the undisturbed medium.
Real image — an image formed where rays of light actually meet. It is inverted and can be put on a screen.
Reflection — the bouncing of a wave off a surface.
Refraction — the change of direction of a wave when it passes from one material into another and its speed changes.
S-wave — a transverse seismic wave produced by an earthquake. S-waves cannot travel through a liquid.
Seismic wave — a wave produced by an earthquake that travels through the Earth.
Transmission — the passing of radiation through a material.
Transverse wave — a wave in which the vibration is at right angles to the direction in which the wave travels. A ripple on a water surface is transverse.
Ultrasound — sound with a frequency above the range of normal human hearing, so above about 20 kHz.
Ultraviolet — electromagnetic radiation with a wavelength a little shorter than violet light. Ultraviolet is ionising and can damage the skin and the eyes.
Virtual image — an image formed where rays of light only appear to come from. It is upright and cannot be put on a screen.
Wave — a disturbance that transfers energy from one place to another without transferring the medium.
Wave speed — the speed at which a wave transfers energy through a medium. It equals the frequency multiplied by the wavelength.
Wavelength — the distance from a point on one wave to the equivalent point on the next wave, measured in metres.