I'm trying to reproduce what you're doing at 2:13, but the center of my sphere is black, and I'm not sure what I'm overlooking. Any ideas?
Hi hhutygags ,
From the way this looks, I'd guess that your Maximum Transmission Bounces are too low (or the Total Max). Maybe you are using the Preset 'Direct Light':
If you switch to the Default Preset, you should be okay. Don't worry, this will be explained later.
If it’s not what Spikey said (it probably is) also check that the material index on your solidify modifier is sourcing the appropriate material.
It could also be a problem with your geometry. See if you have inner faces or flipped normals, maybe even vertices that are not merged and makes your mesh non watertight. Also if that's Eevee, try switching to Cycles and see if the problem goes away. Then at least you'll know if i's a setting in Eevee that needs to be tweaked.