Prof. Muni Mahendra Kumar
The actual phenomenon which was observed was that when the spectra of the light coming from distant galaxies were studied with a spectrometer, it was noticed that the spectral lines are shifted slightly towards the red end of the spectrum.[1] This happens due to what is generally known as ‘Doppler Effect’[2] according to which the light coming from a source that is approaching the observer changes its colour towards the violet end of the spectrum and light from a receding source changes towards the red. Thus, if the galaxies were really receding away from the earth, their light would appear more red. The spectroscopic analysis actually showed a definite shift towards the red end of the spectrum, and hence, it was established that the galaxies were receding from one another, i.e., there is the general uniform expansion of the system of galaxies scattered through the space of the universe.
Footnotes
When the white light of the sun passes through a prism, a colour-band is produced. This is called a ‘spectrum’. In the spectrum, the radiations of the light waves are resolved into their constituent wavelengths. The colours of this spectrum, in order of decreasing wave-lengths, are: red, orange, yellow, green, blue, indigo, violet. Thus, the frequency of the red light is the lowest while that of the violet is the highest.
According to ‘Doppler Principle’ when there is a relative motion between the observer and the source of any wave motion (such as sound, light), the frequency and hence the wave-length of the wave undergoes a change. To an observer approaching the source of wave motion, the frequency appears greater than to an observer moving away. In other words, there is apparent shortening of waves when an observer is nearing their source, and conversely, a lengthening of them when he is receding from their source. Thus light emitted by a receding body would appear more red (red light being of a lower frequency than other colours) than if the body and the observer did not move relatively to each other. A simple example of ‘Doppler Effect’ is the change in the pitch of whistle of a passing train. The whistle appears to be higher in pitch when the train is moving towards an observer than when it is receding from him.
