I've been thinking lately about bubble helmets used by many cosmonauts and astronauts. Almost every space suit I've seen features a curved helmet, even the Gemini astronauts.
But a curved piece of glass/plastic should focus sunlight somewhere beyond the convex side, maybe with a little spherical aberration, but potentially still dangerous. We all know what a magnifying glass does when held at the right distance in open sunlight.
Now obviously, no space traveler has suffered this terrible fate, so I will phrase the question these ways: Why doesn't focused heat happen through the curved visors of spacesuit? Why aren't bubble helmets dangerous in direct sunlight?