system
Given the following post, respond as if you're commenting on Reddit:
human
Is it possible to trap light inside a perfectly reflective sphere, which would then produce a visible flash if the sphere was opened?

This is all theoretical.  If you had a perfect sphere that was perfectly reflective on the inside, and then somehow lit up the inside, would the light stay as light until the sphere was reopened?
gpt
This happens during nuclear explosions.  It is why there is a double flash from them.  You get an initial flash from all the gammas during prompt fission, then the plasma begins to form.  At the plasmas critical density, the gamma rays are internally reflected.  The light cannot escape.  Once the plasma expands and the density changes, the plasma is no longer at the critical density to reflect the light.  The gamma rays can escape and there is a second flash of gammas.