Doppler Effect
Introduction
This chapter covers the biography and discoveries of the Austrian scientist Christian Doppler. It explains terms like 'colour spectrum', 'visible light waves' and 'Doppler effect'.
Who is Doppler?
Christian Doppler was born at the beginning of nineteenth century in Austria. After he finished school he studied and taught mathematics and astronomy in different countries in Europe. In 1842 he wrote his paper called 'On the Coloured Light of Double Stars and Certain Other Stars of the Heavens', illustrating what has since been called the Doppler effect. His ideas led to the theory stating that the universe is expanding. His extensive research in the field of electromagnetism made it possible to follow weather patterns by tracking electromagnetic radio waves.
The Doppler effect (shift)
The Doppler effect, or shift, was named after Christian Doppler, who first discovered it in 1842. The Doppler effect is the change in wavelength of light, radiation or sound from a source due to its relative motion. Doppler thought that waves would have a higher frequency if the source was moving toward the observer and a lower frequency if the source was moving away from the observer.
The Doppler effect can easily be demonstrated with sound waves. When a police car passes by, for example, the sound of its siren changes. The sound waves made by the moving police car are pushed closer together by its motion, so its sound seems to have a higher frequency ('pitch') than it would have if it was parked. In the same way, the sound waves from a receding police car sound as though they have a lower frequency. That effect occurs because of the compression or stretching of waves due to the movement of the source emitting them.
Originally Doppler thought that this effect only applied to sound but later, a scientist called Armande-Hippolyte-Louis Fizeau, generalised Doppler's work by applying his theory not only to sound but also to light. See image 1.
Visible light waves
Visible light waves are electromagnetic waves. Light, sound and electric waves are all waveforms.Electromagnetic waves are caused by an up-and-down motion of electric and magnetic fields. We see these waves as colours. The colour of an object that we see is the colour of light reflected. All other colours are absorbed. Colours make up the colour spectrum.
Colour spectrum
The Latin name spectrum was given by Isaac Newton to the band of colours produced from white light by dispersing it with a prism. A prism is a three-dimensional triangular shape that can split light into its component colours. Newton divided the spectrum into seven colours or 'hues' which are: red, orange, yellow, green, blue, indigo and violet. Even though there are only seven main colours in the spectrum, they all vary continuously from one end to the other. A rainbow is a colour spectrum produced by sunlight passing through raindrops. Each colour has a different wavelength. Red has the longest wavelength and violet has the shortest wavelength. When all the waves are seen together, they make white light. See image 2.
Doppler effect and the universe
The Doppler effect for light waves on Earth is so small that we do not notice it, but when we study starlight the Doppler effect is quite noticeable.
Stars emit light. Using a prism, this light can be spread into a spectrum. If we look at the spectrum of sun light or any other star, we will see the colour spectrum from red through to violet. If a star is moving towards us, the whole pattern of the spectrum is shifted to shorter wavelengths, or towards the blue end of the spectrum. This shift is called blueshift or sometimes violet shift. If the star is receding, the pattern moves to longer, redder wavelengths. This is called redshift.
Edwin Hubble used the Doppler effect to determine that the universe is expanding. After years of scientific research and observation of different celestial bodies through a telescope, Hubble found that the light from distant galaxies was shifted towards the red end of the spectrum. This red shift indicates that stars are moving away from us and from each other. This phenomenon is called the 'expansion of the universe'.
To determine the velocity of the stars, Hubble calculated the compression and stretching of light waves coming from the stars.
Here is the equation:
v = c x (wavelength shift) / (wavelength)
where v is the relative velocity of the star, c is the velocity of light, and the known wavelength of the line as measured in the laboratory.
After analysing a number of stars scientists established that our Sun orbits the centre of our galaxy, the Milky Way, with a velocity of 250 km/sec. Using the Doppler shift, scientists can also tell that stars that are closer to the centre of the galaxy rotate faster. See image 3.
See animation 1.
