Physics

• There is a lot going on in wave motion. A wave changes over time, but it also travels through space
• Types of wave graphs

  • Displacement-time graph
  • Displacement-distance graph
• Displacement-time graph


  Focuses your attention on oscillations at just one point in space
• Displacement-distance graph


  Takes a snapshot in time-shows you the wave shape over the whole space at that instant
• Period


  Time for complete cycle
• Frequency


  1/time for complete cycle
• Amplitude


  Maximum displacement from the central position
• There is no net transfer of matter in wave motion, only the transfer of wave energy
• Displacement


  Physical movement of a particle, or change of another quantity, for example, of electric field for electromagnetic waves
• Oscillation


  Occurs around a mean value of the displacement
• Direction of propagation


  Line along which wave energy travels and in which the wave oscillations propagate that they reproduce the pattern of the wave
• Wave types


  • Longitudinal wave
  • Transverse wave
• Longitudinal wave


  Wave energy travels in the direction of propagation, oscillations back and forth along the direction of propagation
• Transverse wave


  Wave energy travels perpendicular to the direction of propagation, oscillations at right-angles to the direction of propagation - up and down or side to side
• In longitudinal waves there are compressions (regions with particles closer together) and rarefactions (particles further apart) that move with the wave pattern
• Microphone detects pressure variations caused by compression and rarefactions of the air in a sound wave
• Transverse waves in water or in stretched strings can be visible as travelling ripples or as standing waves
• Detectors for transverse waves must be aligned at right angles to the direction of propagation, for example, TV and radio antennas
• Sound is a longitudinal wave
• Radio and TV signals are transverse waves
• Wave speed


  Speed at which a wave transfers energy from one place to another, varies according to the medium through which it propagates
• Wavelength


  Distance travelled by one cycle of the wave
• Frequency


  Number of cycles per unit time
• Wave speed, v, is related to wavelength, λ, and frequency, f, by the equation v = λf
• In musical instruments with strings, you can tune the notes without changing the length by making changes to the medium (the string) and so altering the speed of the waves
• Tension


  Force measured in newtons (N) that affects the speed of waves in a string
• The speed of light in free space (vacuum) is 2.998 x 10^8 m/s
• Nothing has ever been observed to travel faster than the speed of light
• Refractive index


  Ratio of the speed of light in free space to the speed of light in a medium
• The speed of sound in air is fixed for any given temperature and pressure
• You can only tune brass or woodwind instruments by altering the length of pipe being used
• Sound travels in air at 3.0 x 10^8 m/s
• A guitar string of mass per unit length 0.001 kg/m is tuned to give a certain open note. To produce an open note one octave lower (half the frequency), the weight of string used should be four times as much
• The speed of light when travelling through glass of refractive index n = 1.48 is slower than in free space
• Wave-fronts are lines (or planes in 3D) drawn to join points in a wave where all the oscillations are in phase, and are spaced one wavelength apart
• Between two successive wave-fronts, the wave oscillations go through one complete cycle - a phase difference of 360° or 2π rad
• Interference


  Detectable pattern of different strengths (amplitudes) of wave oscillation, caused by waves from different sources crossing and adding together
• Interference patterns only occur when the wave sources are coherent - that is, they have the same frequency and a fixed phase relationship
• No detector is fast enough to directly measure the frequency and phase of light, so interference experiments are used to explore its coherence, and hence to understand the wave nature of light photons
• Diffraction gratings are arrays of regularly spaced slits, designed to break up a plane wave into a set of separate wave sources