I've always been fascinated by light and it's properties- but I've never pursued any form of formal education on the subject beyond the basics that are taught in the calculus based physics series I've taken, so please understand I'm no expert, nor using accurate terminology or descriptions, just trying to share what I understand or believe on the subject.
Basically start with what Patent said- he's got the basics of the subject neatly sumarised.
Essentially "shimmering" is caused by the deflection of the light-wave into pieces that arrive at the location at different times. You're basically getting each "wavelength" difracting to some degree, creating layers of different colored light. So to get the best shimmering, you want a few factors:
-- an intense source of light- this is why halides/LEDs are best for shimmering- you have a small "point" of light that has a high intensity. As the light travels from the point source, you'll have greater separation. With fluorescents, you have an even blanketing of light, and the interference between the light output negates (deconstructive interference) the separation of the light. Thus you see less shimmer when running all your lights then when you're running just a couple. Not sure how the coloration totally comes into play, I do know the best shimmering under T5's occurs with the bluer spectrum bulbs, even more so if they're placed further apart (I get best shimmering when the outside pair of bulbs is lit on my teklight than I do with the inners.)
-- Agitation of the water surface to cause difraction of the light waves. Essentially each different curve of water that the light encounters acts as it's own prism, casting the light upon a new angle. This creation of many angles is what helps us achieve shimmering, especially with less intense/focused light sources. With still water, the light is all roughly bending in the same way, so the light all falls upon the same location regardless of the angle of bending that's being created.
My comments on the more expensive T5's is a reference to the power of the ballasts as well as their reflectors- IME both are vital to getting maximum output of the bulb. While higher quality bulbs should achieve better shimmering (the more pure wavelengths being produced yield the best separation of light, while the lower quality bulbs often have overlapping colors which cause deconstructive interference,) the bulbs are secondary to the intensity of which they are being lit and focused. Obviously pushing more power through the same bulb will create more agitation of the gases inside the bulb and put off more light, hence the more expensive fixtures tending to be visually 'brighter.' I've yet to play around with a PAR meter to find out how much of the increased intensity from the overdriven fixtures is actually usable, but from my experience, it is related.
-=disclaimer=- I wrote this based on my limited understanding and experience, if you see errors, please kindly correct me as I'm obviously still learning, I'm not trying to present any of this as concrete facts, so please also only use it as information of such quality (or lack thereof :xd:)
