- #1
EmilyRuck
- 136
- 6
In the propagation of non-monochromatic waves, the group velocity is defined as
[itex]v_g = \displaystyle \frac{d \omega}{d k}[/itex]
It seems here that [itex]\omega[/itex] is considered a function of [itex]k[/itex] and not viceversa.
But in the presence of a signal source, like an antenna in the case of electro-magnetic wave or a string in the case of sound waves, the actual independent quantity is [itex]\omega[/itex]! And [itex]k[/itex] is a consequence of a vibration of angular frequency [itex]\omega[/itex] which propagates in a certain medium.
With [itex]k = k(\omega)[/itex], we would have
[itex]\displaystyle \frac{dk(\omega)}{d \omega}[/itex]
[itex][\mathrm{seconds} / \mathrm{meters}][/itex] would be the measure units. The velocity could be simply taken as the reciprocal.
Why the dependent ([itex]k = k(\omega)[/itex]) and the independent ([itex]\omega[/itex]) variables has been exchanged in the above definition of [itex]v_g[/itex]? It was just for the sake of measure-units?
If I wanted to find the derivative of - say - [itex]y = \tan x[/itex], it would be quite strange (if not incorrect) to take [itex]dx / dy[/itex]: I would take [itex]dy / dx[/itex] (and then I would use its reciprocal, if I need it). Why here is not so strange instead?
[itex]v_g = \displaystyle \frac{d \omega}{d k}[/itex]
It seems here that [itex]\omega[/itex] is considered a function of [itex]k[/itex] and not viceversa.
But in the presence of a signal source, like an antenna in the case of electro-magnetic wave or a string in the case of sound waves, the actual independent quantity is [itex]\omega[/itex]! And [itex]k[/itex] is a consequence of a vibration of angular frequency [itex]\omega[/itex] which propagates in a certain medium.
With [itex]k = k(\omega)[/itex], we would have
[itex]\displaystyle \frac{dk(\omega)}{d \omega}[/itex]
[itex][\mathrm{seconds} / \mathrm{meters}][/itex] would be the measure units. The velocity could be simply taken as the reciprocal.
Why the dependent ([itex]k = k(\omega)[/itex]) and the independent ([itex]\omega[/itex]) variables has been exchanged in the above definition of [itex]v_g[/itex]? It was just for the sake of measure-units?
If I wanted to find the derivative of - say - [itex]y = \tan x[/itex], it would be quite strange (if not incorrect) to take [itex]dx / dy[/itex]: I would take [itex]dy / dx[/itex] (and then I would use its reciprocal, if I need it). Why here is not so strange instead?