The Use of Waves and Universe Expansion – Sound, The Doppler Effect and Red Shift

The Use of Waves and Universe Expansion – Sound, The Doppler Effect and Red Shift

Sound waves are longitudinal so their vibrations happen in the same direction as the direction in which they’re traveling. They also need to travel through a medium.

When something vibrates it creates a sound:

  • – the greater the amplitude the louder the sound is
  • – the greater the frequency the higher the pitch is

The normal range for a human is between 20 Hz and 20 kHz. However, as people get older this range decreases.

Humans can’t hear above 20 kHz. Sounds within this range are known as ultrasound.


The Doppler effect

The Doppler effect is where there’s a change in both frequency and wavelength.

A good way to think about it is when an ambulance or police car with its siren on drives past. As the noise comes towards you it’s high pitched. However, as the sound travels away from you it becomes low-pitched.

  • When a source travels towards an observer the wavelength observed decreases and the frequency increases.
  • – When a source travels away from an observer the wavelength observed increases and the frequency decreases.



Light is also affected by the Doppler effect as it travels away from an observer.

White light can be split using a prism. The colours, from short wavelength to long wavelength go as follows:

  • – violet
  • – indigo
  • – blue
  • – green
  • – yellow
  • – orange
  • – red

Take the spectrum of light from the Sun. As you can see it’s not a block of colours but contains black lines. These black lines are where light has been absorbed by helium and represents the absorption spectrum for helium.

This is a spectrum of a distant star. It also contains an absorption spectrum. However, it’s different from the Sun’s: the lines have moved to the left. The wavelength of the light is longer; it’s shifted towards the red end of the spectrum hence red shift.

This change is the Doppler effect: their wavelengths have increased and their have frequencies decreased.

It’s been discovered that the further away a star is from Earth the larger the red-shift in its light. This is proof that galaxies far away from us are expanding outwards and that those furthest away are expanding out at a greater speed. On a general level we can conclude that everything in the universe is moving away from everything else.