Sometimes, you want a pretty eye-catching graphical shot of a glass item. It might be a wine bottle, which you want to make anonymous, so you want to light it and avoid showing the label. Of course, you can still light the label, perhaps with a snoot, and just use this technique for dramatic purposes.
I didn't have a wine bottle to hand, but I did have a rather interestingly shaped bathroom perfume thingy. It has blue liquid in the shaped central area, but that won't show unless you light behind it. Same goes for a wine bottle, glass or whatever. You'll get some of the colour from the contents on the edge of the glass, but the centre will usually be pretty dark. There's some physics in there relating to reflective angles, but it's not that important.
So here's my first shot. I placed the perfume glass on a black shiny base, and used my lightbox. This is the exact same initial setup as used in the previous entry for the wristwatch.
I put a 550EX (equivalent to Canon 580EX MkII) flash on either side, lighting most of the diffusers, to create a soft light on each side. This resulted in the following image.
You'll see the top isn't particularly well lit though. On a bottle this wouldn't matter, because it has a top that isn't glass. In this photo though, it's a big deal, because the top edge isn't standing out very well.
I therefore got another flash in the form of a 580EX (equivalent to Canon 580EX MkII). I put it on a lighting stand, and used the lighting stand at an angle, balanced, to support the 580EX above the light box. It's a quick and dirty way to do it, but it works for speed. You can see in the following image, it fills in the edge at the top a bit better.
This is the setup for that shot above.
It's not bad, but the edges aren't all that sharply defined. Ideally the edges will be a thin line.
The further back you move the light source, the steeper the angle of reflection is, and therefore the thinner the edge light shows on the product. So moving the flashes to just light an area at the back of the diffuser, I get the following shot.
You can see how the setup has changed in this image.