Waves 1

• Progressive waves --> A progressive wave is an oscillation that travels through matter (or in some cases, through a vacuum)
• Transverse wave --> In a transverse wave the oscillations or vibrations are perpendicular to the direction of the wave
• Longitudinal wave --> In longitudinal waves the oscillations are parallel to the direction of energy transfer.
• Displacement --> Distance from the equilibrium position in a particular direction.
• Amplitude --> Maximum displacement from the equilibrium position of a wave.
• Wavelength --> Minimum distance between 2 points in phase on adjacent waves
• Period --> The time taken for one oscillation/ one whole wavelength
• Frequency --> The number of wavelengths passing a given point per unit time
• Wave speed --> The distance travelled by the wave per unit time
• The wave equation:

  wave speed = frequency x wavelength

  v = f x lambda
• Phase difference --> the difference between the position of the two waves propagating in the same direction (in a particular time), they don't rise or fall together.
• Absolute refractive index, n :

  • n = (speed of light in vacuum) / (speed of light in material)
  • n = C / V
  • C = 3 x 10^8 m/s
  • the less it slows down the less it refracts
• Snell's law:

  • n1 x Sin theta1 = n2 x sin theta2
  • n x sin theta = constant
• Polarisation:

  • wave property - only occurs in transverse waves
  • only in EM waves
• Total internal reflection ( TIR) --> light occurs at the boundary between two different refractive media.

  • When the light strikes the boundary at a large angle to the normal, it is totally internally reflected. All the light is reflected back into the original medium. There is no light energy refracted out of the original medium.
• 2 conditions are required for TIR:

  1. Light must be travelling through a medium with a higher refractive index as it strikes the boundary with a medium with a lower refractive index. E.g: TIR is possible when light in glass meets air, but not the other way around.
  2. The angle at which the light strikes the boundary must be above the critical angle. This angle depends on the refractive index of the medium
• • At the critical angle C, theta air is 90 degrees
  • nsinC = n(air) x Sin90
  • sin C = 1/n
  • the greater the refractive index the lower the critical angle
• If the angle of incidence is greater than the critical angle then all the light will be reflected back into the original medium by total internal reflection. If the angle of incidence is below the critical angle some of the light will be refracted out of the original medium.
• Fibre optic cables use TIR to transmit information along fibres made from materials such as silica or plastic. The core has a high refractive index compared to the cladding surrounding it. Light can travel long distances without being absorbed because there is very little loss due to absorption or scattering. It also allows signals to be sent quickly over short distances using optical fibres.
• EM waves:

  • Don't need a medium to travel through, can travel through a vacuum
  • Transverse waves
  • EM waves can be reflected, refracted & diffracted
  • As EM waves are transverse waves they can also be plane polarised
  • All EM waves travel at the same speed through a vacuum
  • EM wave equation: e=f x lambda
• Polarisation --> Particles oscillate along one direction only. (E.g: up and down in the vertical direction). The waves are confined to one plane.
• Plane of oscillation --> contains the oscillation of the particles & the direction of travel of the wave. The wave is said to be plane polarised.

  • Light from an unpolarised source, like a filament lamp, is made up of oscillations in many possible planes.
• Longitudinal waves can't be polarised as the oscillations are parallel to the direction of energy transfer. Their oscillations are already limited to only one plane.
• When transverse waves reflect off a surface they become partially polarised. This means there are more waves oscillating in one particular plane, but the wave is not completely plane polarised.
• Unpolarised electromagnetic waves can be polarised using filters called polarisers. The nature of the polariser depends on the part of the EM spectrum to be polarised, but each filter only allows waves with a particular orientation through.
• Intensity --> the radiant power passing through a surface per unit area
• Intensity = power / area
• Intensity & distance:

  • When the wave travels out from a source the radiant power spreads out, reducing the intensity. For a point source of a wave, the energy & power spread uniformly in all directions, that is, over the surface of a sphere.
  • Intensity is inversely proportional to distance
  • I = P/ 4 x pi x r^2
• Intensity & amplitude :

  • Decreased amplitude means a reduced average speed of the oscillating particles
  • Halving the amplitude results in particles oscillating with half the speed, and a quarter of the kinetic energy (Ek = 1/2mv^2)
  • For any wave the intensity is directly proportional to the square of the amplitude.
  • Double the amplitude of a wave & the intensity will quadruple
  • intensity is directly proportional to amplitude^2
• Wave propagation —> the way in which waves travel through a medium or space.