[DoctorBeaver (OP)]

I was watching a program about submarine hunting and, as is common in this sort of program, there was a lot of footage of bombs & depth charges detonating underwater. I noticed there appears to be a small initial explosion followed by a much larger 1. At first, there is a rapidly expanding bright area that reaches a certain limit then stops and is immediately followed by the upwelling of water from the actual detonation.

I was wondering, at first, whether the first "flash" is the visible sign of the explosion with the light only being able to penetrate a certain distance through the water before the actual effect of the explosion becomes apparent by the displacement of the water. However, it seemed to me that the radius of the flash was less than the depth at which it occured, leading me to think that I was wrong as, surely, the light would penetrate the same distance horizontally as vertically through the water.

So, can anyone explain exactly what I was seeing?

[lyner]

The light would be attenuated by the same amount in all directions. How are you judging the range of it at different angles? Why would you expect the distance traveled by the light before becoming too dim to see would be related to the depth of the explosion site?
If you were looking down from an aircraft, you would probably find that the flash would be very bright. If you were looking down from a boat then you might have other things on your mind

The initial brief, high energy, shockwave from the chemical reaction would spread out at the speed of sound in water, then, the production of steam from the energy would produce a longer - lived and bigger volume of 'bubble' which would manifest itself later as it pushed water away over a second or more.
Much later, the bubble of gas plus some steam would find its way to the surface.

Your "rapidly expanding bright area" could be bubbles of dissolved air being shaken out of solution by the passing shockwave. It couldn't be 'light' because it would travel much too fast to see the effect. (200m per microsecond)

[DoctorBeaver (OP)]

Your "rapidly expanding bright area" could be bubbles of dissolved air being shaken out of solution by the passing shockwave

That sounds like a reasonable explanation.

I based my guess of the size on a number of factors - seeing the depth charge enter the water and then the resultant flash below that point, its size relative to a destroyer in frame at the same time, and other such things.

Why would you expect the distance traveled by the light before becoming too dim to see would be related to the depth of the explosion site?

That isn't what I meant. The flash seemed to expand no more than 30-50' yet in many instances the time between the depth charge entering the water and the explosion would indicate that it detonated deeper than that. If it had been the light from the explosion then I would have expected the diameter of the visible flash to be equal to at least the depth of the explosion (as it would propogate at an equal speed in all directions) before the water was seen to be displaced.

[AB Hammer]

DoctorBeaver

 devices that explode need a form of detonation. That would be the pre-flash before the final explosion. Just like dynamite needs the blasting cap to make it explode.   

[DoctorBeaver (OP)]

DoctorBeaver

 devices that explode need a form of detonation. That would be the pre-flash before the final explosion. Just like dynamite needs the blasting cap to make it explode.  

I'm not sure that's the case here. Any pre-detonation would take place inside the container and hence would not be visible. If it ruptured the container enough to have the effect I saw then that would lessen the effect of the explosion quite considerably.

[lyner]

Dr B

The flash seemed to expand

That statement doesn't make sense to me. How could you  see a flash expanding? As I said, it moves at the speed of light. Do you mean you saw an area of illumination? Anything you saw moving would have been at the speed of sound.

The optics of seeing the flash and the various ray paths the light takes to get to you would not be familiar (compared with seeing an explosion in air). I am not sure you are coming to the right conclusion about what you saw. The sea is not like a thin cloud in the sky, which would produce a spherical 'ball of light' around an explosion.
If you put a bulb in the fishpond, (safer than a maroon) you would see the same sort of effect as your underwater flash. Give it a go. (Rubber boots remember)

btw how fast do you think the depth charge falls through the water? In films they don't seem to fall that fast, to me.

[DoctorBeaver (OP)]

SC - I initially said "bright area" and put flash in inverted commas. It was an area that suddenly became much brighter.

Average rate of descent for a depth charge is 10ft/sec. Don't believe everything you see in films  []

[lyner]

OK, so I think you are expecting a symmetrical appearance for the flash. That would not be realistic, I think. Look at fish in a large fish tank at an angle - there is a huge blind spot, which would suggest that the flash would only be visible 'through the water' and not 'over the top'. That would produce the effect that you have seen, I think.
Also, the apparent depth of objects under water is reduced very strongly due to refraction. Actually, perhaps that is the real reason for this apparent illusion. Snell's law works very hard at bending light for very oblique angles i.e. views from a ship or distant aircraft.

[DoctorBeaver (OP)]

It's no illusion; there was a definite brighter area immediately prior to the upward expulsion of water.

[lyner]

Of course. That was the explosive reaction. The 'illusion' was where it appeared to take place. Much shallower  than it actually happened. (if it was actually a light-producing reaction that you saw)
It may just have been the shock wave effect, which would have appeared to spread out on the surface with a strange speed characteristic because it would have a spherical wavefront with centre at the actual explosion. The wave would hit the surface and formed an expanding ring with decreasing velocity.

[DoctorBeaver (OP)]

That sounds like what I saw. It's quite possible that it was shallower than I thought. I only estimated the depth by how long the depth charges were submerged before detonation.

[lyner]

Next time I see a depth charge on the ~TV, I'll look much more closely.
You, too, I expect.

[DoctorBeaver (OP)]

Indeed *checks TV listing for WWII naval films*

[lyner]

Robert Mitchum is a legend.

[DoctorBeaver (OP)]

Good drinker. I once saw him, Oliver Reed and Richard Harris have a session together in the bar at Elstree studios. That was a night to remember. Harris ended up teetotal!

[LeeE]

Just a gratuitous pic of the Dominic Swordfish nuke depth-charge/underwater bomb test from above.  I think it shows the spray dome just before it broke the surface - might be just after though.

[lyner]

Looks to me like the initial spherical shock wave hitting the surface. It's the first thing to arrive and there's no sign of other disturbances. Note the well defined leading edge of the wavefront resulting from the impulsive shock. The white inside the circle must be  bubbles. The sea surface inside the ring still seems to have waves on it like outside.
Have you any idea about the scale of the picture or the height of the plane? If the 'rollers' are 50m spaced, which is possible out in the ocean, it could be a huge circle.
Great picture.

[LeeE]

Yes, it's the sharpness and darkness of the wavefront that leads me to suspect that the ocean surface hasn't been broken yet in that first picture.  Here's another pic of the same test, just after the spray-dome has broken the surface, and you can see how the surrounding surface has clearly foamed at this point, contrasting with the darkness in the first pic.  This pic shows the USS Agerholm (DD-826), which launched the ASROC, in the foreground.  The target raft was 4348 yards from DD-826 and "The rocket missed its sub-surface zero point by 20 yards" (Depth 650ft).

It's difficult to say exactly how big it is at this point.  The link given below says that the spray dome was 3000ft across and rose to 2100ft but it's unclear as to what stage this refers - in some of the other pics on that link, the spray dome gets much bigger as it disperses.



More info and pics from the Dominic test series at:

http://www.radiochemistry.org/history/nuke_tests/dominic/

[DoctorBeaver (OP)]

LeeE - Interesting photos. The white circular area in the aerial photo looks as if it could be the area I was referring to in my question.

This video shows it nicely although it's a nuclearnot conventional explosion.

[LeeE]

Heh - didn't think of trying YouTube.  This one shows the top-down view but the above surface views may be from another test because they're taken from land - the Dominic Swordfish test was supposedly in 17141ft of water.  Although the top-down pic and the top-down vid are at 90 rotation to each other you can match up the clouds and when you compare the sizes relative to the two small clouds closest to the explosion you can see that in the first pic I posted the 'bubble' must still be very deep and not close to the surface at all.

http://uk.youtube.com/watch?NR=1&v=tJkNP7HNHz4