longitudinal and transverse waves

• transverse waves oscillate perpendicular to the direction of energy transfer.

  examples of transverse waves are electromagnetic waves, water waves, or waves on a string.

  you can draw transverse waves on graphs of displacement against distance, or as graphs of displacement against time.
• longitudinal waves vibrate along the direction of energy transfer.

  examples of longitudinal waves are sound waves, and some seismic waves.

  sound waves consist of alternate compressions and rarefactions of the medium it travels through, which is why sound can't travel through a vacuum.

  longitudinal waves are hard to draw graphically, but sometimes they are plotted as displacement against time, where they look like a transverse wave
• polarisation is the restriction of a wave so that it can only oscillate in a single plane. polarisation can only happen for transverse waves.

  before polarisation, the oscillations occur in a mixture of directions, but after polarisation, the oscillations occur in a single plane only.
• polarisation is evidence that electromagnetic waves are transverse
• polarising filters only transmit vibrations in one direction. ordinary light waves are a mixture of different directions of vibration, and they can be polarised with a polarising filter.

  if you have two polarising filters at right angles to each other, then no light will get through
• polaroid sunglasses:

  • they contain a lens with polarising filters with transmission axes that are vertically oriented. this means the glasses do not allow any horizontally polarised light to pass through.
  • when light is reflected from a reflective surface e.g a wet road, it undergoes partial plane polarisation
  • therefore, polaroid sunglasses reduce the glare on the surface of the reflective surface as the partially polarised light will be eliminated by the polarising filter
  • this means objects under the surface of the water can be seen more clearly
• the rods of TV aerials are all horizontal, because TV signals are polarised by the orientation of the rods on the broadcasting aerial. to receive a strong signal, you have to line up the rods on the receiving aerial with the rods on the transmitting aerial - if they aren't aligned, the signal strength will be lower