Topic 4 – Oscillations and Waves4.3 Wave Characteristics
Waves● A wave is an oscillation that can move energyfrom place to place without the transfer ofmatter.● An oscillation moves no energy nor matteroverall.● Waves can either be a pulse or a continuoustravelling wave.● In most cases the oscillations of the particlesmaking up the wave can be considered to besimple harmonic motion.
Types of waves● Transverse waves● The vibration of theparticles are at 90oto thedirection of propagation ofthe wave.● These are seen as waterwaves, surfaceearthquake waves, and aselectromagnetic waves.● Physical trasverse wavescannot propagate ingases.● Longitudinal waves● The vibration of theparticles is parallel to thedirection of propagation ofthe wave.● These are pressurewaves and are seen assound waves andearthquakes.
Intensity and Amplitude● The intensity of a wave is the energy deliveredby it per second per unit area of detector.● It therefore has units of Wm-2● The intensity of a wave is directly proportionalto the amplitude of the wave squared.I ∝ x02
Frequency and Period● The period (T) of a wave is the time taken inseconds for it to make one complete cycle.● As most of the waves that we use have veryshort periods we often use the frequency of thewave instead.● The frequency (f) of a wave is the number ofcomplete cycles in 1 second and is measuredin hertz (Hz).f =1T
Wave Speed● The speed with which a wave transfers energyfrom place to place is known as the wavespeed.● The distance a wave travels in one cycle is thewavelength.● The time taken for one oscillation is the timeperiod.● The wave speed is therefore given by:v=xt= λT=λ f
Waves in 2 Dimensions● There are two distinct ways to describe themotion of waves in 2 dimensions● Rays – The wave is represented by an arrow whichlies in the direction of propagation.● Wavefronts – The wave is represented by a seriesof parallel lines that are perpendicular the directionof propagation and show the position of each wavecrest or compression.● Both of these methods have their uses inspecific situations and can sometimes be seenused together.
Waves in 2 DimensionsA series of wavefrontsA light ray Combined notationThe same wave in 2 dimensions
Transverse Waves in 2 Dimensions● Sometimes, the direction of oscillation of atransverse wave is shown on the ray.Oscillations Up and DownOscillations In and OutOscillations both Up and Downand In and Out
Electromagnetic Waves● Electromagnetic waves rely on the oscillationsof electric and magnetic fields rather thanparticles to move energy.● In a vacuum, all electromagnetic waves travelat the same speed, c = 3.00x108ms-1
Electromagnetic Spectrum● Electromagnetic waves can have anyfrequency.● The continuous spectrum of electromagneticwaves goes from radio waves at low energy upto high energy gamma waves● Different wave bands are divided by theirwavelengths
Questions● An electromagnetic wave has a frequency of5.6x1012Hz. What is the wavelength andwaveband of this radiation?● High energy x-rays have a wavelength of 10pm.What is the frequency of this radiation in avacuum?● A red laser is fired in a vacuum. What is thefrequency of this radiation.● A sound wave is measured to have a frequencyof 15kHz and a wavelength of 2.2cm. What isthe speed of this sound?