Doppler effect

Doppler Effect

Consider a point source of light producing spherical wavefronts, moving away from the observer. Suppose at time t1, the source is at point P. It moves away from the observer and reaches at point Q when time is t2. Thus, the wavefront produced at t = t2 will have to travel longer distance then the wavefront produced at t=t1. As the speed of wavefront is same but the distance is increased, it takes longer time for the later wavefront to reach the observer. Thus, the time difference between arrival of two successive wavefront at the observer is greater than the time difference between emission of two successive wavefronts at the source. The frequency at which the wavefronts are arrived at the observer is less than that of the frequency at which they produced at the source.

The difference in frequency observed by observer and produced by source due to relative motion of observer and source is called doppler effect.

However, in opposite case, if the source is moving towards the observer then the wavefront produced in later time travel less distance than wavefront produced in earlier time. Thus, it takes less time for later wavefront to reach at the observer. Therefore, the observed frequency goes on increasing as the source progresses towards the observer.

Phenomena due to Doppler effect

• Redshift: When a source producing light moves away from the observer the wavelength in the middle of visible region of electromagnetic spectrum gets stretched. Therefore, it appears that light is shifted towards the red end of the spectrum. However, astronomers call increase in wavelength due to doppler effect as red shift. As a result white light appears red.
• Blueshift: When a source producing light moves towards observer, the wavelengths in the middle of the visible region of electromagnetic spectrum decreases. Thus, light appears to be shifted towards blue end of the spectrum. This is called blue shift. Thus white light appears blue.





• Doppler shift: When velocity of source is smaller as compared to speed of light, the fractional change in the frequency is Δ?/? is given by -v_radial/c. This is Doppler shift.
  Δ?/? = -v_radial/c
  Here, v_radial is the component of source velocity along the line joining the observer to the source. v_radial is positive if the source is moving away from the observer. v_radial is taken as negative, is the source is moving towards the observer.
  However, if the velocity of source is greater as compared to speed of light then we need to use Einstein’s special theory of relativity which gives more accurate formula. The doppler effect in astronomy forms the basis for measurements of radial velocities of distant galaxies. Thus, doppler effect proves to be very important.

Keywords: Doppler effect, Red shift, Blue shift, Doppler shift, Huygen’s principle