I was watching this slow motion video of an underwater explosion (1:03 - 1:15) and I noticed that at the beginning of the explosion, it gives off light, then as the sphere of the blast expands the light fades away, but then as the sphere of the blast collapses again the light returns. Why does this happen?
I was only able to come up with one shaky hypothesis: that there might be light tricks going on with the way that the light is refracted by the shape of the sphere, so makes it look like the light fading out, when in actuality it is just being refracted away from the camera.
Of course I am no expert in this and I look forward to being proven wrong.
The explosion creates a bubble of vacuum. The air dissolved in the water disperses ( is sucked) in the vacuum of the initial explosion and gives the visible light by the burning of the explosives. The oxygen is depleted by the combustion and the light stops. As the bubble is reduced by the water pressure after the explosion more oxygenated air enters until it is enough with the residual heat on the fragments to combust again with them.
When you watch the video you see that first a bubble forms, which expands until reaching a maximum volume, and then contracts again until the second flash of light occurs. I think that a possible explanation might be adiabatic compression of the gas created by the explosion. During the explosion, a substantial amount of (thermal) energy is released very fast which creates a very hot gas of subsequent products of the explosive and evaporated water. This high temperature gas has a high pressure and pushes the water out of the bubble space very fast losing temperature during this expansion due to the mechanical work done on the water and due to other energy losses like water evaporation heat, thermal conduction, thermal radiation, etc. When the bubble reaches its maximum radius, a potential energy $U=p·V$ stemming from the original explosion energy E is stored in the bubble. $V$ is the maximum bubble volume, $p$ is the hydrostatic pressure (water plus air above). If the gas cools fast enough, its pressure will become low and the potential energy of U will lead to a compression of the gas, which is adiabatic if fast enough, heating it up so strongly that a short light flash is emitted.