# Waves - Equations and Definitions

wave speed (m/s) = frequency (Hz) × wavelength (m)

Oscillation - a repeated vibration back and forth on the same path, around a fixed equilibrium point.

Travelling Wave - an oscillation which transfers energy rather than matter in radial directions from a source.

Standing Wave - a wave which is set up in a fixed position as a result of the interference of two waves on the same path.

Amplitude - maximum displacement of a point in an oscillation.  Measured from equilibrium line to peak of the wave.

Wavelength - distance between two similar points on a wave, for example peak to peak or compression to compression.

Frequency - number of complete cycles in a given time, usually given as per second or Hertz (Hz).

Time period - time taken for a complete cycle of an oscillation.  Inverse the frequency, usually has units of seconds (s).

Transverse Wave - a wave whose oscillations are perpendicular to the direction of wave energy transfer.

Longitudinal Wave - a wave whose oscillations are parallel to the direction of wave energy transfer.

Compression - an area of pressure higher than the equilibrium pressure.

Rarefraction - an area of pressure lower than the equilibrium pressure.

Sound Waves - a longitudinal pressure wave characterised by compressions and rarefactions.

Oscilloscope - a device which visualises oscillations as waveforms on a screen.  This allows measurement of time period, (and hence frequency) and amplitude.

Echo Location (Pulse-reflection technique.) - A technique used for measuring distance to objects based upon a known speed of a wave and a measured time for a reflected wave pulse to return to the emitter/detector.

Amplify - to increase the amplitude of a wave, e.g. the Voltage of an AC signal or the volume of a sound wave.

Outer ear - roughly cone shaped cartilage which amplifies sound.

Eardrum - a membrane within the ear that vibrates under sound waves.

Ossicles - bones which act like levers to amplify the ear drum vibrations.

Middle ear - that part of the ear which contains the eardrum and the ossicles.

Cochlea - contains small hairs which brush against cells to create an electrical nerve impulse signal.

Auditory nerve - relays signals to the brain.

Inner ear - contains the cochlea and the auditory nerve.

Natural frequency - that frequency which a body vibrates at if it is caused to freely vibrate.

Resonance - a phenomenon where one vibration causes another vibration because the natural frequency of the second body matches the driving frequency of the first.  Energy transfer is very rapid because the so the amplitude of the second increases rapidly.

Spectrum - a continuous range of frequencies or wavelengths.

Electromagnetic spectrum - the full range of electromagnetic wave types.  From longest wavelength to shortest:  radio waves, microwaves, infrared, visible light (ROYGBIV), ultraviolet, X-rays, gamma rays.

Vacuum - an area of space with no particles in it.  (Put in matter)

Thermogram - an image which displays the relative intensities of infrared radiation as colours.

X-Ray scan - a medical scan used for imaging bone and dense tissues as X-Ray radiation is absorbed by dense matter and transmitted through softer tissues.  Only transmitted rays fall on a sensor, (a charge-coupled device or CCD), so a negative image is produced.

Computerised tomography scan - X-Ray scans are taken in layers and so built to a 3D computer image of the patient.

Radiotherapy - use of ionising radiation to kill cells in a tumor.  Rays are focused on cancerous tissue and beam is rotated ensuring healthy cells have only a minimal dose of radiation.

Optics - the branch of physics which considers the path of light and where light will be focussed.

Ray diagrams - a scale drawing representing the path of light.  Used for taking measurements or defining positions in optics.

The Law of Reflection - the angle of incidence will be equal to the angle of reflection, (for waves reflecting from a plane surface.)

Refraction - waves changing direction when it meets a boundary between two mediums at an angle because the light slows down or speeds up.

Snell's Law - sine of the angle of incidence is proportional to the sine of the angle of refraction.  μ = sin i / sin r

Refractive index (μ) - a measure of how much refraction will occur at a boundary.

Dispersion - the spreading out of white light into a visible spectrum by refraction in a prism.

Focus - the point at which light rays meet.

Convex lens / converging lens - a lens which is wider at the centre point than at the edges, it acts to bring rays of light to a point where they will meet.

Concave lens / diverging lens - a lens which is thinner at the centre point than it is at its edges causes light rays to move apart from each other.

Short sight - rays of light focus in front of the retina, can be corrected with concave lenses.

Long sight - rays of light focus behind the retina, can be corrected with convex lenses.

Focal Length - the length between the middle of the lens and the point at which it would focus parallel rays of light.  Measured in metres (m).

Power of a lens - inverse the focal length, has units Dioptres (D).  I.e. if the focal length is 2m the power is 1/2 a Dioptre.

Seismic waves - waves caused by vibrations in the Earth's crust, i.e. earthquakes.

Seismometer - a device to measure seismic waves.

P-waves - stands for both pressure waves and primary waves.  Pressure waves are longitudinal and are faster than S-waves, hence primary as they arrive at seismic listening stations before S waves.

S-waves - stands for both surface waves and secondary waves.  Surface waves are transverse waves, and as they are slower than P-waves they arrive second at the listening stations.

Crust - the solid rocky outer layer of the earth, between 5 and 70km thick.

Mantle - a deep layer of very viscous molten rock.  Nuclear reactions heat this layer.  It is more correct to describe it as solid, but there are gradual convection currents within it so it can be imagined to be a viscous liquid.

Outer core - fluid layer of hot iron and nickel.

Inner core - very dense hot solid iron.