So my question (like most of my questions) is more theoretical then practical. I'm not interested at say 30hz, or building one, but rather what happens at 1Hz or a gigantic transformer at less then 1hz. When does AC cease to become AC and just turn into pulsed DC with no transfer of energy? What kind of magic happens at the extreme ends of the spectrum. IR/Light is just high frequency why can't it work in a transformer (If you think this is a stupid /easy question then you are not thinking about it hard enough i.e. why is glass an insulator of EMF at lower frequencies but a good one at high frequencies if we go past the simple answer of: refraction down fiber optics make it work, or what happens between light and RF)?
Imagine a magic DC to daylight signal gen what would the outputs on the back look like assuming you started at 1hz and cranked it up to 1000THz and noted what happens to your transmission lines as you went up in small 10hz steps.
There appears to be a fundamental misunderstanding of physics here.
J Fourier showed that real signals can be written as the sumation of sinusoids. Hence "pulsed" DC is, in fact, a summation of sinusoids and will therefore be transferable through a transformer.
In order for a transformer to operate in the usual fashion, as a near ideal coupled inductor, there must be a conversion of a field, generated by the wire into a flux, and vice versa at the secondary.
For an efficient conversion of field to flux, a material with a non-unity relative permeability may be used, depending on the freq. range of interest. Similarly, multiple turns may be used in practice instead of a single turn in order to have a greater field for a given current.
I have made transformers at hundreds of MHz - and have bought ones that work even higher. These don't work at low frequencies, as their inductance is insufficient.
At the other end - around VHF to some point at GHz, the ferrite materials become too lossy, the coils self resonate and the currents radiate (== act as an antenna) rather than pass through the wire. The transfomer can not longer be regarded as a monolithic ideal entity, but as a collection of R's, L's, G's and C's, each with a transfer function that is a function of freq - and your transformer equation (and operation) no longer holds.
With the example of glass it can be regarded as a question of boundary conditions (see Maxwell's equations). I believe that you could make a waveguide (== fiber) at 10MHz and use it as a transmission line - but it's just not particularly practical to have a 10m diameter glass rod, instead of a roll of RG-223 coax.
NB glass remains an insulator at light frequencies as well - it is not "conducting", but acts as a dielectric waveguide - the same way as glass can be used as a dielectric medium for making capacitors at low/RF frequencies...