Waves

Level 1

What is a wave?

A wave is a disturbance that propagates in a medium or in space. It carries energy but not matter.

The two main types of waves

  • Mechanical waves: they need a material medium (air, water, rope...) to propagate.
    • Examples: sound, waves on the surface of water, earthquakes.
  • Electromagnetic waves: they can propagate even in a vacuum.
    • Examples: light, radio waves, microwaves. 

  Example: When you throw a pebble into water, the waves that form move away from the impact. It is the disturbance that moves, not the water itself.

Periodicity of a wave

A wave is therefore a disturbance of a medium causing oscillations (vibrations) in that same medium. When this disturbance repeats itself in the same way, then the wave is periodic. We can then easily characterize the wave.

Characteristic quantities of waves

To describe a wave, several quantities are used: 

  • Period ($T$): time taken for a complete oscillation.
  • Wavelength ($\lambda$): distance between two points vibrating in the same way (e.g.: two wave crests).
  • Frequency ($f$): number of oscillations per second.
    • Relationship: $f = \frac{1}{T}$
  • Celerity ($v$ or in the case of light $c$): propagation speed of the wave. 
    • Relationship: $v = \lambda \times f$ 

  Example: A sound wave with a frequency of 440 Hz (the note "A" in music) propagates in the air at about 340 m/s (this is therefore its celerity). 
Its wavelength is: $$\lambda = \frac{v}{f} = \frac{340}{440} \approx 0.77 \, \text{m}$$