Exploring the Ray Model for Radio Waves: Does it Apply to Longer Wavelengths?

[kent davidge]

I have never seen ray model of light being considered for radio waves, or waves of larger wavelengths. I have a feeling that this model does not apply to them. Am I right?

 

[anorlunda]

Do you mean as in ray tracing?

 

[jbriggs444]

I have never seen ray model of light being considered for radio waves, or waves of larger wavelengths. I have a feeling that this model does not apply to them. Am I right?

It is "just" a matter of scale. Light and radio waves are the same thing.

https://en.wikipedia.org/wiki/Line-of-sight_propagation#Radio_horizon

 

[kent davidge]

@anorlunda , @jbriggs444

Let me be more clear,
I have only seen light being described by rays (geometrical optics) when it is visible light. I did not see up until now light of large wavelength, like radio waves, being described by geometrical optics.

 

[anorlunda]

Then you haven't met the physicists on this site. They describe light as waves every day.

 

[Ibix]

A radio telescope dish? Geometric "optics" is probably pretty decent for that application (even if you then make your telescope part of a synthetic aperture telescope). But generally "rays" are useful concepts when wave-y effects like interference and diffraction are negligible - which is pretty often not the case for radio waves.

 

[davenn]

I did not see up until now light of large wavelength, like radio waves, being described by geometrical optics

ummm really ?
you haven't looked very hard

There's no difference to their optical counterparts

 

[A.T.]

ummm really ? you haven't looked very hard

To be fair, the OP doesn't claim to have looked for it at all.

 

[vanhees71]

It is "just" a matter of scale. Light and radio waves are the same thing.

https://en.wikipedia.org/wiki/Line-of-sight_propagation#Radio_horizon

But this scale difference is the key to the answer of the question. Ray optics is an approximation of wave optics for the case that the scale of the obstacles the wave hits is large compared to the typical wave lengths of the em. waves. This is usually not fulfilled for radio waves, which have wavelength roughly in the range of cm to some 100 m.

 

[davenn]

To be fair, the OP doesn't claim to have looked for it at all.

and that is the problem, he should have

 

[sophiecentaur]

This is usually not fulfilled for radio waves, which have wavelength roughly in the range of cm to some 100 m

Usually not but not 'never'. Ray tracing methods for long and medium wavelength radio signals in the ionosphere are about the only way to predict propagation over very long paths. In that case, the dimensions of the propagation path are large compared with the wavelengths involved.

 

[vanhees71]

Sure then the eikonal approximation is valid, but I guess it's invalid to consider how radio waves of long wavelength are reflected and transmitted in presence of usual obstacles around us, which are typically of the size of meters, using ray optics.

 

[tech99]

A particular case of importance is the propagation of radio waves for short distances over the surface of a smooth Earth. There is a ground reflected ray as well as a direct ray, so we have an image of the transmitting antenna below ground. The image is as far below ground as the actual antenna is above. The two rays now add at the receiving antenna and in most cases result in near cancellation. For communication above about 30MHz, for instance, it is found that the inverse square law does not hold and the received power is proportional to about 1/(distance^4).