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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.

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  • $\begingroup$ Maybe it has something to do with sonoluminiscence, the emission of light from bubbles in water created by strong acoustic waves.. $\endgroup$ – freecharly Oct 14 '16 at 20:55
  • $\begingroup$ One of the slow-mo guys comments on this phenomenon at 5:40 and suggests that the explosion gases might be re-igniting. This might be a question of chemistry rather than physics. $\endgroup$ – sammy gerbil Oct 14 '16 at 23:23
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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.

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  • $\begingroup$ How do you explain the creation of a vacuum by an underwater explosion. Isn't it just hot gas of the explosive and hot water vapor created by the release of energy? $\endgroup$ – freecharly Oct 15 '16 at 23:47
  • $\begingroup$ @freecharly Put "vacuum" in quotes. It tries to become vacuum, which is the space between atoms and molecules which is filled up by gases etc, and part of the oxygen is already in the aeriated volume of water, BUT it is at a lower pressure than the water around it and will be getting difusion of air from the surrounding water with its oxygen. At the end of expansion the oxygen is depleted and collapses again until by difusion more air with oxygen comes in ( pressure is equalized when bubble goes to zero volume). $\endgroup$ – anna v Oct 16 '16 at 5:50
  • $\begingroup$ This is an interesting picture. I was thinking more about it and was wondering whether the light flash might be caused by adiabatic compression of the bubble gas due to the potential energy stored in the water bubble. I tried to give an alternative answer along these lines. $\endgroup$ – freecharly Oct 17 '16 at 3:09
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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.

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