We have the transmission helper which is used by the glTF extension KHR_materials_transmission which can do a better job with refraction, but it’s not available to users. This is because we’re not yet sure of the best way to handle refraction, so we’d rather not make things public that we’ll have to put up with forever if it’s not the right solution…
You can try searching for “transmission helper” in the forum, maybe something interesting will come up…
Also ask @PatrickRyan if he has any tips for simulating a lens effect.