Nature of waves

Nature of waves

1. Wave type

   • Mechanical vs electromagnetic
     • Electromagnetic
       • created when charged particles are accelerated, producing a wave of continuously varying electric and magnetic fields
         • consist of an electric and a magnetic field which oscillate perpendicular to one another, with the electric field oscillating in the plane of the source
       • all electomagnetic waves are transverse
       • travel at the speed of light: 3 x 10^ ms-1
       • do not require a medium for transmission i.e can travel through a vacuum
       • Electromagnetic spectrum
         • Visible light wavelength - 4-7 x 10-7
         • Uses
           • Radio - radio-telescopes in space
           • Microwave - heating food in microwaves
           • Infrared - thermal imaging devices
           • Visible - photosynthesis in plants
           • UV - flourescent lamps to detect forged bank notes
           • X-ray - used in medicine for diagnosis and therapy
           • Gamma ray - used in medicine to destroy tumours
     • Mechanical
       • Oscillations
         • repeated/periodic motion around a fixed mean position
         • e.g pendulum swing, masses on a spring.
         • Properties of an oscillation
           • Amplitude - the maximum displacement from the mean position (metres, m)
           • Frequency - the number of complete oscillations per second (Hertz, Hz)
             • Time period = 1 / frequency
           • Time period - the time taken for one complete oscillation (seconds, s)
       • require a medium for transmission, i.e cannot pass through a vacuum.
       • generated by vibrating sources, the energy from which is transmitted by the particles within it
         • Note: it is energy that is transferred, particles oscillate around a fixed mean position.
       • may be longitudinal or transverse
   • Longitudinal vs transverse
     • Longitudinal waves - particles oscillate parallel to the direction of energy transfer
       • e.g sound waves
       • made up of compressions are rarefactions
     • Transverse - particles oscillate perpendicular to the direction of energy transfer
       • e.g water waves, electromagnetic waves
       • made up of peaks and troughs
   • Circular vs plane
   • continuous vs train
     • continuous - no definitive start and end
     • train - definite start and end
       • pulse - no repeated up and down motion, one wavelength

2. The wave equation

   • speed = frequency x wavelength
   • derived from v = s/t

3. Graphs

   • Displacement-time graphs
     • the graph will be sinusoidal
     • Phase difference - the fraction of a cycle between two paticles in one wave or two seperate waves.
       • Phase - the position within a cycle that a particle occupies relative to the onset of the cycle
         • Two points are in phase if they are in corresponding points of an oscillation.
           • If two waves are half a cycle out of phase, they are said to be antiphase
           • points which are in phase have a whole number of wavelengths between them
       • 2 pi radians or 360 degrees is equivalent to one whole oscillation
         • waves which are antiphase have a phase diffference of pi radians/180 degrees.
           • points which are antiphase have a distance of n wavelengths / 2 between them
   • Distance-displacement graphs
     • differs from a distance time graph, which shows the movement of a single particle within a wave as time progresses, as it shows the instantaneous position of all the particles along a section of the wave.
       • The vibrations of particles along a wave are all slightly out of phase with their neighbouring particle
       • For longitudinal waves, the positive displacement is taken as movement to the right of the mean position, and negative as movement to the left.
     • wavelength - the distance between two adjacent positions that are in phase (metres, m)

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