Question

We can easily hear the Doppler shift of a car passing by but we do not notice the Doppler shift of light from its headlamps. Why is that?

Answer 1

We know that formula for doppler shift (for sound) is given by:

Where, V  is the speed sound in air.

V_ois the speed of observer.

V_sis the speed of source.

f is the frequency of sound of source.

This case, we can easily feel the doppler shift.

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This equation is also applicable for light.

But the term that will change is V , which will be the speed of light = 3 x 10⁸ m/s

Hence, V_s <<< V and V_o <<< V

So, V-V_s ≈ V

and V+V_o ≈ V

So the equation will become:

Hence, we will not feel any frequency shift for light. (unless the car or source or both are moving at a speed similar to light, which you know, will not be possible !)

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