blue glow?

[YT2095]

I`m sure quite a few have seen pictures either on TV or Physics text books, of Radioactive material being submersed in huge water tanks and there`s a strange Blue Glow at the bottom.

 

What is that?

 

I think it would be safe to say it isn`t a Plasma as it`s under water.

[swansont]

I`m sure quite a few have seen pictures either on TV or Physics text books' date=' of Radioactive material being submersed in huge water tanks and there`s a strange Blue Glow at the bottom.

 

What is that?

 

I think it would be safe to say it isn`t a Plasma as it`s under water.[/quote']

 

Cerenkov radiation

 

When a particle moving through some medium (index of refraction, n, is >1) travels faster than the speed of light in that medium (c/n), it emits the radiation. Similar in some respects to the shock wave from breaking the sound barrier.

[YT2095]

EwWwWwWw )

 

cool stuff! Thanx man, you`re a star

 

I`m glad it added the part about the Vacuum though, I don`t want to start another FTL frenzy again

it`s a beautifull blue color though, I`de love to see it 1`st hand with my own eyes.

[swansont]

EwWwWwWw )

 

cool stuff! Thanx man' date=' you`re a star

 

I`m glad it added the part about the Vacuum though, I don`t want to start another FTL frenzy again

it`s a beautifull blue color though, I`de love to see it 1`st hand with my own eyes.[/quote']

 

I'm blushing.

 

Yes, it's very important to note that the particles are travelling c/n < v < c in this phenomenon. Faster than light but not faster than c, because it's not in a vacuum.

[Skye]

Interesting. In the first link it says that When a charged beta particle moves through water it tends to "polarize" (orient) the water molecules in a direction adjacent to its path, thus distorting the local electric charge distribution. After the beta particle has passed, the molecules realign themselves in their original, random charge distribution. A pulse of electromagnetic radiation in the form of blue light is emitted as a result of this reorientation... and then in the wikipedia it says that As a charged particle travels, it disrupts the local electromagnetic field in its medium. Electrons in the atoms of the medium will be displaced and polarized by the passing EM field of a charged particle. Photons are emitted as an insulator's electrons restore themselves to equilibrium after the disruption has passed.

 

They sound similar, however it seems that the first says that the beta rays change the alignment of the molecule, and the second that the beta rays affect the location of electrons in atoms.

[Johnny5]

Cerenkov radiation

 

When a particle moving through some medium (index of refraction' date=' n, is >1) travels faster than the speed of light in that medium (c/n), it emits the radiation. Similar in some respects to the shock wave from breaking the sound barrier.[/quote']

 

I don't understand this swansont. I thought that light could only travel slower than c inside a medium. If n is greater than one, then c/n is LESS than c, so what did you mean?

 

Also, explain this shock wave sound barrier thing to me.

 

(You know if it's not too much trouble)

[swansont]

They sound similar' date=' however it seems that the first says that the beta rays change the alignment of the molecule, and the second that the beta rays affect the location of electrons in atoms.[/quote']

 

The betas affect the electronic alignment, which is the same as saying they polarize them or affect the location of electrons.

[swansont]

I don't understand this swansont. I thought that light could only travel slower than c inside a medium. If n is greater than one' date=' then c/n is LESS than c, so what did you mean?

 

Also, explain this shock wave sound barrier thing to me.

 

(You know if it's not too much trouble)[/quote']

 

Read post #4.

 

Then Google on "sonic boom." I'm no expert on the topic. Perhaps someone else is up on the details...

[YT2095]

AFAIK a sonic "Boom" is the resultant of air finding it`s own natural place again after being displaced by a projectile that travels faster than it`s ability to do so.

 

in this case the projectile would be the Beta particle, and the air would be the water and associated particles in the atoms that make up the H2O.

 

at least that`s the way I understand it (however simplified).

 

edit: perhaps a better way would be to think of Clapping your hands, that works on compression, but IF your hand were already together and you could move them appart FASTER than the air could fill the space between then, when the air DID fill that gap, it would would make a rather loud noise also.

 

sort of an Inverse Clap

[Johnny5]

Read post #4.

 

 

 

Ok' date=' I've read number four and I don't get it.

 

You say "faster than light" but "not faster than c"

 

I just don't understand.

 

I am reading the article on Cerenkov radiation as well.

 

For whatever it is worth, I am trying to get it.

 

 

[b']Do you mean that the speed of the beta particles in the rest frame of the reactor are travelling faster than the speed of light in the water?[/b]

[YT2095]

light travels slower in a medium such as air and water and crystals and such.

light can only reach it`s TRUE speed in a vacuum. that speed is known as `c` and accepted as the universal standard.

[Johnny5]

 

the particles are travelling c/n < v < c in this phenomenon.

 

Faster than light but not faster than c' date=' because it's not in a vacuum.

 

[/quote']

 

Particles travelling at c/n... hmm

 

[math] c = 299792458 \frac{m}{s} [/math]

 

dc/dt = 0

 

In this case, n is the index of refraction of water. Let me find it on google.

 

Here is a link that has it to two decimal places Index of refraction of water

 

[math] n_{water} = 1.33 [/math]

 

[math] \frac{c}{n_{water}} = \frac{299792458}{1.33} = 225407863 \frac{m}{s} [/math]

 

I didn't bother rounding off to two decimal places.

 

You say

 

c/n < v

 

 

What is v?

[Johnny5]

AFAIK a sonic "Boom" is the resultant of air finding it`s own natural place again after being displaced by a projectile that travels faster than it`s ability to do so.

 

Can you be more specific.

 

I am thinking of a missile moving through the air. I realize that the nosecone of the missile pushes the air away from where it was, as the missile moves, and then after the missile goes a ways futher, the air which it displaced "falls back" to where it was before the missile passed through. Yes I get this very much, it's intuitive.

 

So right when the missile reaches Mach 1, there is a Mach cone. Do you know anything about that YT2095?

[YT2095]

a little yes, but don`t expect formula

 

have you ever seen a boat on the water and it leaves a bow wave (wake) in the shape of a `V`

or fired a riffle round into ballistics gell and looked at the displacement patern?

 

as you can see there is a pressure being built up at the front of the moving object (incidently that pressure is used for lift in an aircraft wing, an aerofoil explots this difference).

 

that "built up" pressure must come at a sacrafice from somewhere, it must create a Negative pressure (badly worded but ok for simplicity sakes).

if the projectile is moving FASTER than the air in the neg pressure zone can fill its `V` wake, then it collapses (like clapping your hands together), that in turn makes a loud noise.

 

basicly the air slams back into possition again at such a velocity (like an Implosion) that it "drags" Extra air in with it, that air then stabilised to normal pressure again but sends out a "Ripple" (like throwing a stone in a pool), that ripple eventualy hits your ears.

 

I may come up with a better way to explain later, but it`s the best I can do for now off the top of my head

[Johnny5]

 

I may come up with a better way to explain later' date=' but it`s the best I can do for now off the top of my head [/quote']

 

That was great. Thank you

[YT2095]

[Johnny5]

that "built up" pressure must come at a sacrafice from somewhere' date=' it must create a Negative pressure (badly worded but ok for simplicity sakes).

if the projectile is moving FASTER than the air in the neg pressure zone can fill its `V` wake, then it collapses (like clapping your hands together), that in turn makes a loud noise.[/quote']

 

Just like a clap. I get it.

 

There is a V.

 

the apex of the letter V could be the nosecone of a ship. The sides of the letter V are the 'wake' the ship is making in the air.

 

When the ship is moving slowly through the air, the sides of the letter V close gently, after the ship moves by.

 

But if the ship moves sufficiently fast through the air, the sides of the letter V will "smack together" just like hands clapping.

 

Marvelous analogy YT, thanks.

[Martin]

you can tell the speed of the particles from the angle the light goes off at

 

called Cerenkov angle (its cosine is the ratio of the speed of light in the medium divided by the speed of the particles causing the glow)

 

say you have a beam of elecrons going thru a tank of water in a certain direction

and say the blue light diverges off from that beam-direction by 30 degrees

(the directions the rays of cerenkov light go make a cone)

 

then you can calculate the speed of the electrons from that (its a useful way to measure it and people use this relation about the cerenkov angle all the time)

 

anybody want to say what the speed is?

[Johnny5]

you can tell the speed of the particles from the angle the light goes off at

 

called Cerenkov angle (its cosine is the ratio of the speed of light in the medium divided by the speed of the particles causing the glow)

 

say you have a beam of elecrons going thru a tank of water in a certain direction

and say the blue light diverges off from that beam-direction by 30 degrees

(the directions the rays of cerenkov light go make a cone)

 

then you can calculate the speed of the electrons from that (its a useful way to measure it and people use this relation about the cerenkov angle all the time)

 

anybody want to say what the speed is?

 

You have to go slow with me. Sorry. I am doing my best to comprehend a universe you know.

 

The angle the light goes off at? I can tell the speed of the particles' date=' from the angle the light goes off at? Ok it has a name Cerenkov angle. Let me google it and see what happens.

 

I am reading this now.

 

I am especially intrigued by this part here:

 

As in sonic booms and bow shocks, the angle of the shock cone is inversely related to the velocity of the disruption. Hence, observed angles of incidence can be used to compute the direction and speed of a Cherenkov radiation producing charge.

 

I need that part explained to me.

[swansont]

What is v?

 

The speed of the particle.

[Martin]

I am reading this now.

.

 

good. you are reading the right thing

Swansont is the guide in this thread and he will make sure you get to the right destination

 

the blue (and UV etc) light is diverging off from the direction of the beam by the angle of 30 degrees

and the medium where this happens is water where light goes 0.75c

so what is the speed of the particles in the beam?

 

I have to go but i think you can immediately answer this (and swansont can verify if you need confirmation)

[Johnny5]

The speed of the particle.

 

I've read enough to understand this much.

 

You have water in the reactor.

So speeds are being defined in the rest frame of the reactor.

 

So in this frame, we have photons moving through the water.

 

They will have the following speed in the reactor frame:

 

c/n

 

Where c=299792458 m/s, and n = 1.33 (the index of refraction of water)

 

But apparently, there are electrically charged particles moving through the water as well.

 

And these charged particles have a speed through the water which exeeds the speed of photons through the water.

 

Which means that if the charged particles are accelerated to c/n in the reactor frame, and then surpass c/n in the reactor frame, there is a mach cone formed, similiar to when a jet fighter reaches the speed of sound in air.

 

So thats what you meant when you said v> c/n.

 

V is the speed of the electrons through the water

c/n is the speed of some photons through the water.

 

Alright I understand the diagram in the wikipedia article somewhat.

 

The red line represents the velocity vector of some electron which is moving through the water, which is at rest in the reactor frame.

 

Now, as this beam electron moves in this frame, there is an associated magnetic field with it. I guess that B field influences the atomic electrons in the atoms of the water.

 

And apparently if the speed of the beam electron exceeds c/n, then there is some kind of shock wave thing happening, Mach cone thing, which leads to all that blue cerenkov radiation which a human eye can see.

 

The blue lines in the diagram represent the blue photons.

 

I followed the geometry, and I see that

 

[math] cos \phi = \frac{1}{n \beta} [/math]

 

Where b is v/c.

 

I assume that beta is the Cerenkov angle.

 

So we can write:

 

[math] cos \phi = \frac{1}{n \beta} = \frac{c}{nv} [/math]

 

c/n = speed of light through dielectric medium.

 

So we can write the formula as follows:

 

[math] cos \phi = \frac{1}{n \beta} = \frac{v_l}{v} [/math]

 

Where V_l represents the speed of light in a medium, in this case water.

 

 

the blue (and UV etc) light is diverging off from the direction of the beam by the angle of 30 degrees

and the medium where this happens is water where light goes 0.75c

so what is the speed of the particles in the beam?

 

 

[math] cos \frac{\pi}{6} = \frac{v_l}{v} = \frac{.75 c}{v} [/math]

 

[math] cos \frac{\pi}{6} = \frac{\sqrt {3}}{2} = .866025... [/math]

 

As can be checked with a handheld calculator. Hence:

 

[math] .866 = \frac{.75 c}{v} [/math]

 

It therefore follows that:

 

[math] v = \frac{.75 c}{.866} [/math]

 

And c = 299792458 meters per second' date=' so

 

[math'] v = \frac{.75 (299792458)}{.866} = 259627884 [/math]

 

It was supposed to come out faster than c.

 

The only thing I can figure is that phi = 60, not 30. I guess you meant q = 30, where:

 

[math] 180 = 90 + \phi + \theta [/math]

 

If I use 60, instead of 30, i get:

 

[math] cos 60 = \frac{1}{2} = \frac{.75 c}{v} [/math]

[Martin]

 

It therefore follows that:

 

[math] v = \frac{.75 c}{.866} [/math]

 

And c = 299792458 meters per second' date=' so

 

[math'] v = \frac{.75 (299792458)}{.866} = 259627884 [/math]

 

It was supposed to come out faster than c.

 

The only thing I can figure is that phi = 60, not 30.

 

I think you did it right!

I think that 2.596E8 meters per second is right.

 

the electrons are going 0.866c

and light in water is going 0.75c

[Johnny5]

I think you did it right!

I think that 2.596E8 meters per second is right.

 

the electrons are going 0.866c

and light in water is going 0.75c

 

 

Isn't that a matter for experiment to decide?

 

 

I gotta go grab something to eat, I'll be back later.

 

Thanks for everything Martin.

[Martin]

hold your horses

you did it right the first time

the angle was 30 degrees

 

the speed of the electrons is not supposed to come out bigger than c

 

it is supposed to come out bigger than the speed of light in the medium

which in this case is the speed of light in water which is 0.75c