yafray caustics glass material question

I believe I used caustics the right way. Is it used for making glass materials? Is it used to refract light? Anyway, my question:

I add a UVSphere. I apply my glass material to it and put the right settings into the yafray panel. When I render, only the default blue background appears. I also did some testing with a white plane beneath the sphere. The sphere did not appear at all. Then, I added a cube around the sphere. THat’s what made it work. Why does it do this? I don’t know all the physics of this lighting and stuff but I know it has something to do with how light reflects off of stuff, and I guess there just wasn’t anything to reflect off of?

Refraction comes in two varieties in CG - look at my answer to https://blenderartists.org/forum/viewtopic.php?t=39375 - check that caustics is really what you’re after.