Comments by "TheEvertw" (@TheEvertw) on "Underwater Explosions at 120,000fps - The Slow Mo Guys" video.

About your question at 5:35: 'whatever is inside it'. The banger produces a certain amount of hot gas, which is what causes the light. That is generated suddenly, at high pressure. That high pressure spike pushes the water away in a spherical bubble. So initially, the bubble is filled with gas. The water around the bubble is accelerated by the gas pressure. As the bubble expands, the pressure in the bubble decreases, causing its temperature to drop. Thus it stops emitting light. At some point, the pressure in the bubble drops below the ambient pressure, and will start to decelerate the waterfront around the bubble. But due to the amount of kinetic energy in the water, it can not stop immediately. The bubble will continue to expand even as the pressure in the bubble drops to near vacuum, but the speed of the water is decreasing. At some point, the water comes to a stand still. At that point, the bubble is at its maximum, and inside is a near vacuum. The water will be drawn back in to fill the vacuum. As that happens, the pressure inside the bubble will rise again, heating up the gas inside. At some point it starts to glow again, just before the bubble collapses. During the internal collisions as the bubble collapses, most energy of the blast is dissipated. Thus we do not see a second spherical bubble. What remains is an irregular 'cloud' of combustion gasses, at near-ambient pressure. This cloud is pushed to the surface by the water it displaces. So the short answer is: the bubble is filled with a modest amount of combustion gasses. When the bubble is at its maximum, the pressure inside is near 0 Pa (absolute).
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Pratyush Kishore True, that is what near vacuum means ;-) I have no idea how low it drops, but it'll be quite low.
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@bukster1 Sonoluminescence seems to also be caused by heat as a bubble collapses, though the final word on that has not been spoken.
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@C.j-Upside3 As a gas expands, it cools down. That is called adiabatic cooling. Conservation of energy applies, so when a gas is compressed again to the same volume, it gets almost the same temperature as before -- i.e. it starts to glow again. Light is usually radiated by heat, i.e. a red-hot iron, an old-fashioned light bulb, or hot gasses like in the sun. It is called incandescence or thermal radiation. There are some other physical processes that can create light, e.g. electrons jumping between specific energy levels like in LED's, but that is not what is happening here.
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@EdenTheCreator366 Having had to type it all, I'd say you're right ;-)
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@SF-fb6lv This is a typical oscillation where static and dynamic energy are exchanged. The situation switches repeatedly between all-static energy and all-dynamic. There are four moments of interest: 1). All-static with the largest bubble size: no movement, low pressure and temperature inside the bubble. 2). All -dynamic, with all energy in the movement of water, directed towards the center and ambient pressure inside the bubble. 3). All-static with the smallest bubble size, no water movement and very high pressure and temperature inside the bubble. 4). All-dynamic with movement away from the center. (turbulence makes this more complex, causing movement even at moments of low dynamic energy. But to understand we must simplify) At its largest expansion, the static energy is stored in the water displaced by the bubble. The actual pressure inside can be estimated by dividing the volume of the gas at ambient pressure by the volume at its greatest expansion (not taking temperature in consideration). As volume expands with the cube of the cord, that pressure will be quite low (i.e. a bubble twice the diameter will have 8 times the volume).
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