Quantisation of light and Doppler effect

[Vibin Narayanan]

Light consists of quanta(small packets of energy). Then how do we explain doppler effect of light the same way we do for sound? What is the valid explanation of light doppler effect which is consistent with the photon picture?

 

[DrClaude]

Light consists of quanta(small packets of energy). Then how do we explain doppler effect of light the same way we do for sound? What is the valid explanation of light doppler effect which is consistent with the photon picture?

The frequency of the photon (and hence its energy) is frame-dependent.

 

[Vibin Narayanan]

The frequency of the photon (and hence its energy) is frame-dependent.

That's it? I simply didn't get it. Energy changing with the motion of source or observer or both, doesn't violate conservation of energy?

 

[DrClaude]

Energy is conserved, but it is dependent on the frame. For example, I will measure a ball moving relative to me as having kinetic energy, but in a frame co-moving with the ball, it will have none.

 

[Drakkith]

That's it? I simply didn't get it. Energy changing with the motion of source or observer or both, doesn't violate conservation of energy?

Nope. Consider yourself in a car moving down the road. If another car comes up behind you 1 mph faster than you're going and taps your bumper, your car is not totaled, despite the fact that you both may be traveling 70+ mph.