# Doppler Effect of light ; and electric , magnetic fields

by A Dhingra
Tags: doppler, effect, electric, fields, light, magnetic
 P: 196 my query is : The electric and magnetic fields don’t squeeze or stretch then how is Doppler Effect of light possible? In the phenomenon of Doppler Effect, light emitted from a moving source is detected to have different frequency. If this is taken on terms of detecting the no. of waves passing through the detector in one second it is fine. But according to the principle of relativity, the speed of light is a constant …. It does not change even if its source is moving…. And that means if still Doppler Effect is observed then the frequency, say, has got increased, then the wavelength should decrease to keep the speed of light constant. And as the wavelength appears to decrease though the original emitted one is larger….. it can be visualized as the wave has got squeezed due to its motion. But electric and magnetic fields don’t change their structure even when they are not stationary, i.e., they don’t stretch or squeeze….. Then how is the phenomenon of Doppler Effect of light observed?
 Mentor P: 17,529 What does it mean for a field to squeeze or stretch? What has squeezing or stretching to do with the Doppler effect? I don't understand the terms you are using or the connections you are trying to make.
 P: 196 by squeeze and stretch , i mean the shape and structure of the field lines which don't change. .... when the electric charge is in motion, its motion does not affect the shape of the fields, that is, the field is dragged with the charge without getting the distance between the consecutive field lines change.( though that distance is already very small or negligible...) and the connection is that ,as they remain unaffected, how is Doppler effect possible which calls for the wavelength to change...in a way causing the field lines to get affected by motion........
Mentor
P: 17,529
Doppler Effect of light ; and electric , magnetic fields

 Quote by A Dhingra by squeeze and stretch , i mean the shape and structure of the field lines which don't change. .... when the electric charge is in motion, its motion does not affect the shape of the fields
This is incorrect. The motion of a charge very much affects the shape of the fields that it generates. The equation governing that is called the Liénard–Wiechert fields:

http://en.wikipedia.org/wiki/Li%C3%A...hert_potential
http://fermi.la.asu.edu/PHY531/larmor/index.html

As you can see, the Lienard Wiechert fields are not at all the same as the static field given by Coulomb's law.

In the end, the Doppler effect for light happens the same way as the Doppler effect for sound. To first order, if a wave of frequency f travelling at c is emitted by an object travelling at v then the distance between successive peaks is not c/f but (c±v)/f and therefore the received frequency for a stationary receiver is different from f. Do you understand the Doppler effect for sound?
 P: 56 the doppler effect at very high speeds from the moving object was seen in bodies orbiting around black holes. I cant find the article i was think of but try this link http://videolectures.net/mit801f99_lewin_lec23/

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