Waves; Polarisation, Reflection and Refraction

By Fintan Darragh

The very basics of waves are pretty simple to grasp for A-Level, especially if you’ve already got a GCSE in physics.

Waves

• Waves originate from vibrations.
• Complex sounds are made from the addition of numerous sinusoidal waves.
• Amplitude is a non-negative scalar measurement of the maximum displacement of a wave.

v = fλ

This is the fundamental wave formula, and will help you to find the speed of the wave, the frequency or the wavelength.

• v = the speed of a wave (ms^-1)
• f = the frequency of the wave (Hz)
• λ = the wavelength (m)

Transverse waves

• The ultimate example of a transverse wave is light.
• In transverse waves, particles of the medium or of the field oscillate at right angles to the direction of travel of the wave.

Longitudinal waves

• The very best example of a longitudinal wave has to be sound.
• Longitudinal waves make particles in the medium oscillate along the direction the wave is travelling in.

Polarisation

• Only transverse waves can be polarised.
• Polarisation restricts the oscillation of a transverse wave to one plane at a right angle to the direction of travel.
• A transmitter and a receiver aerial should be correctly aligned if the signal is polarised in order to receive the signal properly.
  • This is why you have to fiddle about with your aerial to pick up different signals better
  • If your receiver isn’t aligned properly with the transmitter, the waves will only hit the aerial at one part, and not up and down the entire aerial, so you won’t pick up anything.

Reflection

Reflection is easy to remember. When a wave is reflected off of something, the angle between the normal and the incidental wave is the same as the angle between the normal and the reflected wave.

Refraction

Refraction is a little trickier than reflection.

• Refraction occurs when light (or any wave) passes from one medium into another with a different optical density.
• It causes the light to bend either towards of away from the normal, depending on the optical density difference.
  • If the wave is moving into a more dense medium (e.g. air to water), the light will bend towards the normal.
  • If the wave is moving into a less dense medium (e.g. water to air), the light will bend away from the normal.

And finally…

• Energy is party transmitted and partly absorbed at an interface.
• Energy is partly absorbed by transmitting media.
• You spell it polarisation, not polarization.

• Test to see how well you’ve done with some of our GCSE physics tests, especially ones on the units, the properties of waves and light and sound.
• Work on your waves calculations with AS physics-level questions and answers.