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I've been looking at underwater explosions from various US navy shock trials:

e.g. https://www.youtube.com/watch?v=yV0eyP4nO20 and https://www.youtube.com/watch?v=PS2whGDzmzg

If you go frame by frame through the explosions you see a rapidly spreading cyan blue effect:

Bright blue shockwave?

At first I thought this may be a cgi thing the navy added in, but you can see it also even in videos from the 90s:

90s era explosion

I assume this also rules out any possibility of it being a digital camera artifact.

So what's going on? How is it so blue? It's clearly more saturated a colour than anything else in the image, including the sky. On video it appears to spread, so it doesn't seem to be the explosion itself somehow filtered through the water. I'm assuming it is a physical phenomenon, maybe it's bio-luminescence, but would that look like that?

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    $\begingroup$ This might be the result of sonoluminescence. $\endgroup$
    – Ruslan
    Jul 21, 2023 at 12:08
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    $\begingroup$ Don’t forget shock waves forming lots and lots of bubbles. $\endgroup$
    – Jon Custer
    Jul 21, 2023 at 15:48
  • $\begingroup$ Further to @JonCuster's observation: light interacting with lots and lots of small bubbles will undergo Rayleigh scattering, preferentially sending the blue light to large angles. With a spectrometer you might be able to determine the scatterer size distribution from the light spectrum, but you (probably) can't do that from an RGB video. $\endgroup$
    – rob
    Nov 28, 2023 at 16:56

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I don't have the full picture of what's going on, but here are my thoughts. There are several facts to consider:

  1. The color of the glowing patch is greenish-blue at the beginning, becoming navy blue closer to the end of the glow;
  2. The glowing patch expands in about 70 ms to the length of USS Gerald R. Ford.

The greenish-blue color could be explained by absorption of red light by water, which in particular is easy to see in more regular conditions: via icebergs (image source):

This would explain the appearance of an underwater "something" at the beginning, like in this frame:

But this doesn't explain the navy-blue color at some stages of the explosion, like in this frame (though there is the greenish color on the rim of the patch here):

Now, knowing the length of the ship (333 m) and having estimated the time the patch expands to this length (a bit more than 2 frames of a 30 FPS video), we can compute the speed of increase of the diameter of the patch:

$$\frac{333\,\mathrm{m}}{2\,\mathrm{frame}/(30\,\mathrm{frame/s})}=5\,\mathrm{km/s}.$$

A single side of the patch moves at half this speed, $2.5\,\mathrm{km/s}.$ This is much higher than the speed of sound in water, about $1.5\,\mathrm{km/s},$ so it's either the shock wave, or the wall of the hot gas bubble. A video of a very small-scale (aquarium-size) underwater explosion was made by the Slow Mo Guys, where you can see the behavior of the hot gas bubble. A screenshot from their video:

From the above mentioned video by Slow Mo Guys I conclude that the shock wave itself is not visible. Moreover, it wouldn't slow down as it expands, while the glowing patch in the navy trial explosion does. So most likely the light comes from the bubble. It's quite surprising for me that the bubble would be so huge, given our usual assumption of incompressibility of water, but I don't really know what size should be expected from an 18-kiloton explosion (which is the energy used by these trials).

So, although the above analysis doesn't explain everything, my current hypothesis is that the glowing patch is the representation of the wall of the hot-gas bubble.

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It's light from the explosion. When regular light hits the water, parts of it reflects of the surface giving the same wavelength that comes from the sun, clouds etc. Parts of the light also gets absorbed by the water, except for blue light (characteristic of water). But this blue light is obscured by the reflection.

The light of the explosion illuminates the water bellow the water, without contributing to any reflection. Think of it as a blue filter in front of a flashlight. Since the water around the explosion gets illuminated by a greater amount of light than the water further away it looks more saturated.

Edit: I mistakenly attributed the blue color to a band gap instead of the wavelengths that water does not absorb. Though with this change in my answer I feel it explains it much the same.

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    $\begingroup$ Water doesn't have a band gap, it's not a crystal. $\endgroup$
    – Ruslan
    Jul 21, 2023 at 12:01
  • $\begingroup$ Water doesn't have a band gap, but it does absorb longer wavelengths. Blue light is least absorbed. $\endgroup$
    – mmesser314
    Jul 21, 2023 at 14:53
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    $\begingroup$ This doesn't seem very plausible to me. The cut-off between the blue and not-blue is very sharp, and in the video the area seems to expand. $\endgroup$
    – Fhnuzoag
    Jul 21, 2023 at 16:49
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could this be 'Cherenkov radiation' the blue color the cone of light both matches the description of this effect. https://www.youtube.com/watch?v=_Kf2f_9MfPc

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    $\begingroup$ For elementary particles to overcome light speed in a water, which is $\approx 225~000~km/s$, you need atomic warheads explosion under water, which briefly would spawn superluminal particles in the water, thereby producing Cherenkov radiation. But this is highly unrealistic,- that warship shock tests would use these types of weapons, banned for tests by international treaties. So certainly, NO. $\endgroup$ Nov 28, 2023 at 11:31

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