Polaroid photos are seemingly a magic trick, and in reality there is something of an illusion in the way it “develops before your eyes.”
After you take an instant photo, it passes through two stainless-steel rollers, which spread chemicals that are collected in a blob at the edge of the plastic film sheet.
These chemicals start developing the photo, but the rollers also do something you wouldn’t likely imagine, they spread a layer of “opacifier,” which is a thick, white, opaque coating that keeps light from reaching the photosensitive layers below.
At this point, the photo underneath has already developed, yet you still can’t see it. That’s because the opaque layer is still in place. It’s only after the photo is completely developed below that acid seeps up and reacts with the opacifiers that cover it.
When that happens, the opaque coating slowly becomes translucent, then transparent, slowly revealing the photo that’s already fully developed below.
So while it looks like the photo’s developing before your eyes, all that’s really happening is that a layer that keeps you from seeing the photo is slowly disappearing.