Waves

Waves

1. Waves and vibrations

   • Types of waves
     • A wave is when the vibration of a source periodically disturbs nearby particles
     • Mechanical waves are waves that need to have a substance to pass through
     • Electromagnetic waves are waves that do not need to have substance to pass through
   • Longitudinal waves
     • Longitudinal waves are waves in which the direction of vibration of the particles is parallel to (along) the direction in which the wave travels
       • Examples of longitudinal waves are: primary seismic waves and sound waves
   • Transverse waves
     • Transverse waves are waves in which the direction of vibration is perpendicular to the direction in which the wave travels
       • Examples of transverse waves are: Electromagnetic waves and secondary seismic waves
   • Polarisation
     • Transverse waves are plane-polarised if the vibrations stay in one plane only.
     • If the vibrations change from one plane to another, the waves are unpolarised.
     • Longitudinal waves cannot be polarised
     • If unpolarised light is passed through a polaroid filter, the transmitted light is polarised as the filter only allows through light which vibrates in a certain direction.
     • If unpolarised light is passed through two polaroid filters, the transmitted light intensity changes if one polaroid is turned relative to the other one
       • The filters are said to be crossed when the transmitted intensity is a minimum. At this position, the polarised light from the first filter cannot pass through the second filter, as the alignment of molecules in the second filter is at 90° to the alignment in the first filter.
     • Light is part of the spectrum of electromagnetic waves. The plane of polarisation of an electromagnetic wave is defined as the plane in which the electric field oscillates.
     • Polaroid sunglasses reduce the glare of light reflected by water or glass. The reflected light is polarised and the intensity is reduced when it passes through the polaroid sunglasses.

2. Measuring waves

   • Key Terms
     • The DISPLACEMENT of a vibrating particle is its distance and direction from its equilibrium position
     • The AMPLITUDE of a wave is the maximum displacement of a vibrating particle. For a transverse wave, this is the height of a wave crest or the depth of a wave trough from its equilibrium position.
     • The WAVELENGTH of a wave is the least distance between two adjacent vibrating particles with the same displacement and velocity at the same time.
     • One complete CYCLE of a wave is from maximum displacement to the next maximum displacement.
     • The PERIOD of a wave is the time taken for one complete wave to pass a fixed point.
     • The FREQUENCY of a wave is the number of cycles of vibration of a particle per second, or the number of complete waves passing a point per second.
       • The unit of frequency is hertz (HZ)
       • For waves of frequency f, the period of the wave = 1/f
   • Wave speed
     • The higher the frequency of a wave, the shorter its wavelength.
     • Crests of straight waves travelling at a constant speed in a ripple tank.
       • Each wave crest travels a distance equal to one wavelength (λ) in the time taken for one cycle.
       • The time taken for one cycle = 1/f, where f is the frequency of the waves.
       • Therefore, the speed of the waves, c = distance travelled in one cycle / time taken for one cycle
         • c = λ/(1/f)
           • c = fλ
   • Phase difference
     • The phase of a vibrating particle at a certain time is the fraction of a cycle it has completed since the start of the cycle.
     • The phase difference between two particles vibrating at the same frequency is the fraction of a cycle between the vibrations of the two particles, measured either in degrees or radians, where 1 cycle = 360° = 2π radians
       • The phase difference in radians = 2πd/λ

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