[DoctorBeaver (OP)]

Well, the phenomenon is real. So what's the definitive answer as to what causes it?

[LeeE]

I wonder if it's because you can't see the light coming out in other directions until it's scattered by something i.e. the bubbles caused by the shockwave, just as you can't see a laser beam in the air unless it's scattered by mist or smoke particles.  You'd then appear to see the light spread as the shockwave expands.

[lyner]

Shock wave and bubbles,  afterwards. Then a load of gas and steam etc.
I think the initial flash of light would be too short and intense to see it progressing in any way. Of course, you only see light which is coming in your direction - directly or indirectly.

[LeeE]

There would still be a fireball emitting light, even though it's underwater.

[lyner]

Of course. But all you could hope to see is a flash - no details of 'movement' of light. You couldn't, for instance, photograph the wavefront.

[LeeE]

Although the initial flash would be very quick, being an explosion, I think the resulting fireball, where the heated gases are still producing light, lasts long enough to illuminate the expanding shockwave.

[lyner]

Very possibly.
You'd have to be there! to see it. Keep your eyes open for films in the near future. Then we can resurrect this thread and speak from an informed position> (Now that would be novel).

[LeeE]

No TV here, so films are out, and film FX explosions use petrol/gasoline anyway, which aren't really proper explosions - too slow.  Explosion vids on youtube are probably a better bet.  This one is interesting because you can see the shockwave.

http://uk.youtube.com/watch?v=Q_y7JLJGX_w

[lyner]

This is good, too:
http://www.youtube.com/watch?v=16heorrfsgY&feature=related

The shock wave is very impressive but there is no visible fireball. I am surprised, considering that it is blinding in atmospheric tests.

[LeeE]

If you look very carefully - by clicking quickly on the play button I was almost able to step through it frame by frame - you can just see the fireball emerging from the surface of the water before it's obscured by spray and debris.

[lyner]

Well spotted.
It must have been very brief (if you needed to step frame) compared with surface explosions which last for a long time on films. Film is better for showing this because eyes / tv can flare for a long time, whatever the period of the actual flash.

[LeeE]

Have a look at the pics of the Redwing Seminole test - you'll need to scroll down a bit.

http://nuclearweaponarchive.org/Usa/Tests/Redwing.html

The Wilson Effect cloud is interesting.  On that sequence of pics you can see the fireball but it's quickly obscured by the cloud, even though the tank it was in was relatively small.

[yor_on]

Light slows down in 'matter' right.
So how much would light slow down in 'bubbles?

Under all the time the device explodes there should be light produced, shouldn't it?
From the 'blasting cap' to the burnables driving the 'explosion'.

So how fast will those bubbles be created.
If they are there before the explosion 'burnt through' 
Wouldn't it be possible for light to be 'reflected' in that foam and so keep light confined for some moment?
Also, light from above reflected back from foams surfaces will give a white effect I believe?
As the bubbles will spread the reflective spectrum, much the same as colored glass becomes 'white' when crushed.

[lyner]

YES
Except for the 'light slowing down in the bubbles' bit. It would go faster through the air in the bubbles than through the water. Lots of internal reflections would diffuse the light.
I am surprised that there is so much absorption, though. The Wilson Effect Cloud, apparently.

[DoctorBeaver (OP)]

This thread has got a lot more interesting than I thought it would. Thank you all.