Physics Topic 3 - Waves

Physics Topic 3 - Waves

1. Properties of waves

   • Waves can be described as oscillations or vibrations about a rest position
   • Electromagnetic waves cause oscillations in electrical and magnetic fields. All electromagnetic waves are transverse
   • Parts of a wave
     • Rest positions- the undisturbed position of particles or fields when they are not vibrating
     • Displacement- the distance that a certain point in the medium has moved from its rest position
     • Peak- the highest point above the rest position
     • Trough- the lowest point below the rest position
     • Amplitude- the maximum displacement of a point of a wave from its rest position
     • Wavelength- distance covered by a full cycle of the wave. Measured from peak to peak or trough to trough
     • Frequency- the number of waves passing a point each second
   • Mechanical waves
     • Mechanical waves can be longitudinal or transverse
     • Mechanical waves cause oscillations of particles in a solid, liquid or gas and must have a medium to travel through. Once the wave has passed the medium ends up back in its rest positions
   • Longitudinal
     • In longitudinal waves the vibrations are parallel to the direction of wave travel
     • Examples include: sound waves, ultrasound waves, seismic P-waves
     • Longitudinal waves show areas of compression and rarefraction
       • Compressions are regions of high pressure due to particles being close together
       • Rarefactions are regions of low pressure due to particles being spread further apart
   • Transverse
     • In transverse waves the vibrations are at right angles to the direction of wave travel
     • Examples include: ripples on the surface of water, vibrations in a guitar string, electromagnetic waves, seismic S-waves
   • Wave period and wave speed
     • Time period = 1 / frequency
     • Wave speed = frequency x wavelength
     • Sound is a mechanical longitudinal wave. The wave is passed on by collisions between particles so the speed the wave moves depends on the density of the particles
     • When sound moves from one medium into another, the change in speed will also cause a change in wavelength, frequency is unaffective
     • Constant frequency means that the wavelength will be proportional to the wave velocity
     • Wave speed = distance / time
   • Sound waves
     • Sound waves are longitudinal. They cause particles to vibrate parallel to the direction of wave travel.
     • Sound wave vibrations can travel through solids, liquids or gases. The speed of sound depends on the medium through which it is travelling
     • Sound cannot travel through a vacuum because there are no particles to carry the vibrations

2. Reflection and refraction

   • Transmission and absorption
     • Waves arriving at the interface (boundary) between two materials can be reflected, transmitted or absorbed
     • Absorption examples:
       • Waves hitting the beach get absorbed by the sand
       • Sunlight landing on the face is absorbed, warming the skin
       • Sound waves hitting thick curtains give up their energy and the sound is muffled
     • Transmission examples
       • Sea waves passing a shallow area continue with their energy mostly unchanged
       • Light passing through a glass window continues with over 95 of its energy
       • Ultrasound waves scanning a baby pass from flesh into bone and continue with enough energy for the machine to detect the echo
   • Refraction
     • Waves transmitted at the interface (boundary) between two materials may change direction. Refraction is the change of a wave at such a boundary. All wave types can be refracted
     • The density of a material is one of the factors that affects the speed that a wave will be transmitted through it
     • Faster - away, slower - towards
   • Reflection
     • Law of reflection: angle of incidence = angle of reflection
     • The angles of incidence and reflection are measured between the light ray and the normal (90 degrees to the surface)

3. Sound and ultrasound

   • Longitudinal
   • The vibrations can travel through solids, liquids or gases
   • Sound waves enter the ear canal and cause the eardrum to vibrate. Three small bones transmit these vibrations to the cochlea. This produces electrical signals which pass through the auditory nerve to the brain, where they are interpreted as sound
   • The frequency of a sound wave is related to the pitch that is heard: high frequency sound waves are high pitched, low frequency sound waves are low pitched