system
Given the following post, respond as if you're commenting on Reddit:
human
Reading about bhangmeters and the 'double flash' that occurs with a nuclear detonation. Question inside...

The wiki article has this line in it:

This signature, with a double intensity maximum, is characteristic of atmospheric nuclear explosions and is the result of the Earth atmosphere becoming opaque to visible light and transparent again as the explosion's shock wave travels through it.

Can someone explain that statement?
gpt
The shock wave from the explosion ionizes air molecules, and basically turns air into plasma. Photons can't pass through plasma very well, as they like to interact with charged particles, and so for a short time, all light emitted by the blast is trapped inside the shock wave sphere. After the shock wave has passed, the ions recombine into atoms and molecules quite quickly, and the gas becomes transparent again. The shock wave also dilutes away, so after a few seconds it's not able to ionize enough particles to turn the gas entirely opaque.