The general types and properties of WAVES - an introduction

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

INTRODUCTION

Be able to describe that waves transfer energy and information without transferring matter.

When your TV receives the signal its just coded data in the electromagnetic wave, no material substance arrives!

However, energy itself must be transmitted or no effected could be produced by the TV receiver!

Similarly, when ripples on water cause floating objects to bob up and down, energy is needed to do this, but neither the floating object or the water itself actually move in the direction of the transverse waves.

The most dramatic transfer of energy involves earthquake waves, both transverse and longitudinal, yet the effects are transmitted and felt miles from the epicentre and no part of the earth's crust moves in the direction of the seismic waves but it may move violently from side to side, up and down or compressed/decompressed.

When sound waves vibrate your ear drum no air moves from the TV, person or musical instrument etc., yet energy is transferred through the medium of air, otherwise, what causes your ear drum to vibrate!

You need to know the two formulae for waves relating them to wavelength, frequency, speed and distance travelled.



  • Know the differences between longitudinal and transverse waves by referring to sound, electromagnetic and seismic waves.
    • You need to understand that in a transverse wave the oscillations are perpendicular (at 90o) to the direction of energy transfer, but in a longitudinal wave the oscillations are parallel to the direction of energy transfer ie direction of forward wave movement.

    • Shaking a slinky spring from side to side produces a transverse wave, as ripples on water and all electromagnetic radiation.

    • Pulling and pushing on a slinky spring produces pulses of energy transmitted as a longitudinal wave like a sound wave travelling through a medium ie the 'compressions' and 'rarefactions' are in the same direction as the wave movement.

    • Seismic waves e.g. from earthquakes (form of tectonic activity) can be of both types of wave.

General properties of waves

 

The above diagram gives an idea of a transverse wave where the oscillations are at 90o to the direction the wave moves

eg electromagnetic radiation, ripples-waves on water, shaking slinky spring from side to side

  • You should know, understand and be able to use the terms frequency, wavelength and amplitude of a wave in terms of this diagram.

    • The top of the wave form is called a crest and the bottom of the wave is called the trough.

    • The wave amplitude = distance from the baseline of zero displacement to the point of maximum displacement (to top of crest or to bottom of trough)

    • One wavelength (m) = distance of one complete cycle or oscillation = horizontal distance from any point on the wave until where it begins to repeat = distance between two crests = distance between two troughs

    • The frequency of a wave (Hz) = number of complete cycles/oscillations per second = number of complete waves passing a given point per second. 1 Hertz, x Hz = x oscillation/s

 

The above diagram shows the compression and decompression (rarefaction) of a longitudinal sound wave, illustrated by the pushing pulling of a slinky spring, the diagram also illustrate what happens to the ground with one of the types of earthquake wave (the compressional P waves, which go right through the Earth!).

The oscillations in a longitudinal wave are in the same direction as the wave is moving

Know and understand that longitudinal waves show areas of compression and rarefaction (diagram above illustrates longitudinal sound waves, wave B has twice the frequency and half the wavelength of wave A).

  • a) Know and understand that waves transfer energy.

  • b) Know and understand that waves may be either transverse or longitudinal.

    • You need to understand that in a transverse wave the oscillations are perpendicular (at 90o) to the direction of energy transfer, but in a longitudinal wave the oscillations are parallel to the direction of energy transfer ie direction of forward wave movement.

    • Shaking a slinky spring from side to side produces a transverse wave, as ripples on water and all electromagnetic radiation.

    • Pulling and pushing on a slinky spring produces pulses of energy transmitted as a longitudinal wave like a sound wave travelling through a medium ie the 'compressions' and 'rarefactions' are in the same direction as the wave movement.

    • When your TV receives the signal its just coded data in the electromagnetic wave, no material substance arrives!
    • However, energy itself must be transmitted or no effected could be produced by the TV receiver!
    • Similarly, when ripples on water cause floating objects to bob up and down, energy is needed to do this, but neither the floating object or the water itself actually move in the direction of the transverse waves.
    • The most dramatic transfer of energy involves earthquake waves, both transverse and longitudinal, yet the effects are transmitted and felt miles from the epicentre and no part of the earth's crust moves in the direction of the seismic waves but it may move violently from side to side, up and down or compressed/decompressed.
    • When sound waves vibrate your ear drum no air moves from the TV, person or musical instrument etc., yet energy is transferred through the medium of air, otherwise, what causes your ear drum to vibrate!
  • Know that electromagnetic waves are transverse (radio, microwave, infrared, visible light) as are waves on water - eg ripples, sound waves are longitudinal and mechanical waves may be either transverse or longitudinal.

  • Know that all types of electromagnetic waves travel at the same speed through a vacuum (~ empty space) - 'the maximum speed of light' which is 3 x 108 m/s.

  • Know that electromagnetic waves form a continuous spectrum.

    • You should know the order of electromagnetic waves within the spectrum, in terms of energy, frequency and wavelength.

    • You should appreciate that the wavelengths vary from the minute 10-15 metres for extremely high frequency gamma rays to more than 104 metres for very low frequency radio waves.

  • Know that waves are reflected and refracted at boundaries between different materials and can spread out when passing the end of a barrier or through an opening.

  • Know and understand that light waves can be reflected, refracted and diffracted.

  • Waves are readily reflected off smooth flat surfaces e.g. light reflected off a smooth surface like a mirror.

    • Reflection

      • The vertical dotted line is called the 'normal', it isn't a ray, but helps in the construction and interpretation of ray diagrams. A plane mirror means one of a perfectly flat surface.

      • Angle 1 = angle of incidence of incident ray. Angle 4 = angle of reflection of the reflected ray.

      • Angle 2 = Angle 3. All angles are measured with respect to the 'normal' which is at 90o to the surface.

  • To explain how waves will be refracted at a boundary in terms of the change of speed and direction, we need a diagram!

  • The above diagram illustrates the phenomena of refraction by considering what happens to waves eg visible light.

  • You can think of the parallel lines as representing a series of crests of waves eg think waves on the sea or ripples in a pond on throwing a stone in.

  • Refraction A: When waves from a less dense medium, hit a boundary interface, and enter a more dense medium, the waves 'bend towards the normal' ie refraction occurs.

    • This happens because on entering the more dense medium, the waves slow down causing the change wave in direction.

    • The obvious examples you see in optics experiments are light rays passing from air into more dense transparent plastic blocks or triangular and rectangular glass prisms.

  • Refraction B: When waves from a more dense medium, hit a boundary interface, and enter a less dense medium, the waves 'bend away from the normal' ie refraction occurs.

    • This happens because on entering the less dense medium, the waves speed up causing the change in wave direction.

    • The obvious examples you see in optics experiments are light rays emerging from transparent plastic blocks or triangular and rectangular glass prisms, and passing out into less dense air.

    • When you observe an object half in water and half in air e.g. poking a stick into still water, you see a 'bent' distorted image, because, the light rays from the object and bent at the air-water interface because of refraction.

  • If the waves hit the interface at an angle of 90o (perpendicular) to the interface between the two mediums, there is still a change in speed and wavelength, but there is NO refraction and the wave frequency remains the same.

  • Diffraction is the effect of waves spreading out when passing through a gap or passing by a barrier. In effect, waves go round corners! and it doesn't matter if its sound, light or water waves - they all diffract and bend round corners! The effect is so small with light (tiny wavelength), you don't notice it, but you see water waves bending around objects at the seaside and you can hear sounds from round a corner.

  • You should appreciate that significant diffraction only occurs when the wavelength of the wave is of the same order of magnitude as the size of the gap or obstacle.

  • A: There is a relatively small diffraction effect when waves pass through a wide gap that is much bigger than the wavelength of the wave.

  • B: You get the maximum spreading or diffraction when the waves pass through a gap of similar size to the wavelength of the incident waves.

  • You can see these effects with transverse water waves at the seaside as waves hit the protective walls of a harbour BUT you need a very tiny slit to observe diffraction with light waves because of their tiny wavelength.

  • Be able to use both the equations below, which apply to all waves (and their rearrangements):

    • appropriate units used in ()

    • a) wave speed (metre/second, m/s) = frequency (hertz, Hz) x wavelength (metre, m)

      • in 'shorthand'    v = f x λ

        • rearrangements:  f = v / λ   and   λ = v / f

    • b) wave speed (metre/second, m/s) = distance (metre, m) / time (second, s)

      • in 'shorthand'    v = d / t

        • rearrangements:  d = v x t   and   t = d / v

      • This is the general formula for the speed or velocity of anything moving.

    • (a), (b)

    • Note that you are not required to recall the value of the speed of electromagnetic waves through a vacuum ...

      • .. is very big, 'speed of light' = v = 3 x 108 m/s

      • Be able to do examples of calculations using the wave speed formula and its rearrangements.

 


  • Check out your practical work you did or teacher demonstrations you observed in Unit P1.5, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

    • reflecting light off a plane mirror at different angles,

    • using a class set of skipping ropes to investigate frequency and wavelength,

    • demonstrating transverse and longitudinal waves with a slinky spring,

    • carrying out refraction investigations using a glass block,

    • carrying out investigations using ripple tanks, including the relationship between depth of water and speed of wave,

    • investigating the range of Bluetooth or infrared communications between mobile phones and laptops,

    • and demonstrating the Doppler effect for sound.


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