Why is it that radio waves spread out in proportion to the square of the distance, while higher frequency electromagnetic waves, like microwaves, infrared waves, light, etc are able to propagate as beams? What fundamental property allows higher energy waves to travel differently than lower energy?
[Physics] Why do radio waves spread out while higher frequency waves travel in beams
electromagnetic-radiationelectromagnetism
Related Solutions
Photons interact with matter if the matter offers quantum transitions that match, or nearly match, the photon's energy in the inertial frame of the matter. Ordinary matter such as wood, stone, etc. offers several groups of possible quantum transitions.
- Rotation of molecules (if they are free to rotate, i.e., not condensed matter)
- Vibration of molecules - bending, quivering actions
- Electronic excitations
- Nuclear excitations (there being various kinds, ignored here for simplicity)
Microwaves have such low energy they can't do much, though they might excite some types of vibrations on larger floppier molecules - however, any type of molecule that could be described as "floppy" probably isn't good for construction materials. Rotational modes aren't possible in a strong material made of crosslinked polymers or silicates. So microwaves mostly fly right through.
Near-infrared and visible light can kick electrons into higher molecular orbitals. Even if the energies aren't a match, just close, there is interaction, as Heisenberg lets them cheat temporarily. Also, having more energy, visible light photons can stir up a greater variety of vibrational modes. There's nothing in common wall materials to prevent that, and in fact, the interaction with photons is so strong that the material, if not super-thin (microns), will be opaque. Of course, glass is an exception.
Gamma rays are of such high frequency, electrons (or ions, or polarized ends of molecules) can't keep up due to inertia - so no interaction, or only a little. At the right frequencies, gamma photons can interact with nuclei, but for a randomly chosen source of gammas, its photons are unlikely to match closely enough with any of the available nuclear excitations, and can't really do much at the molecular level - therefore, the material is almost transparent.
All this is so oversimplified...
Higher energy gamma and longer wavelength radio?
Keep in mind that the different 'kinds' are merely human labeling conventions for a spectrum that is continuous in the mathematical sense. There is no feature of "radio" that distinguishes it objectively from microwaves. We just pick a boundary on the basis of some technological limitations that apply when we decide the difference and stick labels on.
The reason there aren't labels beyond radio and gamma is that there is no real need to label those bands.
Best Answer
Due to diffraction, wave effects become more important as the size of the wave source becomes comparable to the length of the wave. Visible light has micrometer-scale wavelengths, so a millimeter-sized light source is thousands of wavelengths across and diffraction isn't a very big deal. But radio wavelengths can be many meters, producing similar collimation for a radio "beam" would require an emitting antenna hundreds or thousands of kilometers across.
You can use the same logic to think about shadows. A hair that's less than a millimeter across can cast a well-defined shadow, while radio waves diffract around buildings. However larger objects can cast well-defined radio shadows: for instance astrophysical radio sources disappear when they are covered by the Moon or the Sun, which are both very many wavelengths across.
Note that even "collimated" light undergoes dispersion. Any sort of focusing optical system will produce a beam waist at some finite distance from the final focusing element (mirror or lens or whatever); beyond that beam waist the intensity of the light falls off like $r^2$ just as if a light source were at that location. A perfectly collimated beam of light is prohibited by the uncertainty principle, unless the beam is infinitely wide.