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Author Topic: Lasers and noise.  (Read 5333 times)

Offline Merlin

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Lasers and noise.
« on: 10/05/2003 05:45:51 »
I have a question.  Lasers are just light, correct?  Therefore, many science fiction movies have been chastised because the y have lasers that make sound.  But, a lightning bolt makes sound because it is so extremely hot cuasing the air around it to expand.  So, would a laser capable of dealing significant amounts of damage to an object make a crak of thunder type sound?
« Last Edit: 13/05/2003 10:07:33 by Exodus »


 

Offline chris

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Re: Lasers and noise.
« Reply #1 on: 10/05/2003 13:33:03 »
Hi Merlin

great question and raising a point that I'd never thought about before. A trip to a Pink Floyd concert, or listening to the printer on the desk, confirms that 'ornamental' lasers don't make sounds, because they are not sufficiently powerful to produce the same local heating effects as a bolt of lightning.

During a thunderstorm the 'flash' usually comprises a discharge in the region of 100 million volts which heats the air in its path to over 33,000 degrees celsius (60,000 F). Air heated by the discharge expands very rapidly, producing a pressure wave that we call thunder, which propagates outwards in all directions at the speed of sound (about 760 mph).

Lasers are used to produce local heating effects in fusion experiments so presumably if you could build a laser that could deliver the same energy as a lightning bolt to the surrounding air then it too would produce 'thunder'.

Maybe one of the physics gurus - ?Dickie - could share some of their wisdom on this one ?

Chris
 

Offline Dickie

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Re: Lasers and noise.
« Reply #2 on: 12/05/2003 16:40:49 »
Lightning is essentially a static discharge. A high enough electric field will cause air to ionise, allowing a current to pass through. As Chris said, this causes quite a lot of heating.

Laser light is simply an e-m wave that doesn't really interact with the air it passes through. Even the light from the sun doesn't cause the air to be heated significantly, so a directanalogy to a lightning bolt would be almost impossible.

Lasers do cause heating when they hit something that absorbs light, however. A powerful pulse from a laser will cause intense heating at the target, and will produce not only a loud bang, but a shock wave powerful enough to knock someone off their feet.

This was covered a few months back in New Scientist, but I've not been able to find the article online. The intention was to make a non-lethal riot-control weapon.


Dickie.
 

Offline nilmot

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Re: Lasers and noise.
« Reply #3 on: 12/05/2003 19:49:55 »
If laser is just light being intensified can you see it? It's still light right? Or is it coloured?

Tom
« Last Edit: 12/05/2003 19:50:25 by nilmot »
 

Offline Exodus

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Re: Lasers and noise.
« Reply #4 on: 12/05/2003 20:51:50 »
lasers are used for the precision cutting of metal, especially aluminium in the aircraft industry. Computers guide the lasers from a template.

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Offline Dickie

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Re: Lasers and noise.
« Reply #5 on: 13/05/2003 13:14:34 »
Tom -

Yes, laser light is just light. All the little photons (light particles / wave packets) are the same as any other light. The colour depends on the material used in the laser, but is usually visible these days. Early devices (MASERs) used a non-visible wavelength.

Unless you put in a lot of effort with smoke, you won't see the beam itself, just the spot where it lands.

The difference is the way in which the light is produced. In a lightbulb, a metal filament is heated by passing a current through, and it gives off photons as and when it feels like it, in whatever direction it wants. A laser produces coherent light- all the photons go in the same direction, with the same frequency, and are in phase (they all wiggle at the same point)

/-\_/-\_/
-\_/-\_/-    not in phase
/-\_/-\_/

/-\_/-\_/
/-\_/-\_/    in phase
/-\_/-\_/

I hope the ASCII art makes sense.

Before the '60s, "ray guns" were thought to be impossible, becuase to heat up the target, the gun would have to get at least as hot to produce the ray. Because "lasers" (in Dr. Evil voice) don't use heat to make light, they don't have this problem, and can make light intense enough to cut metal, as Chris pointed out. This is also why you can look at a 100W lightbulb, but not an AA battery powered laser pointer.


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Offline nilmot

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Re: Lasers and noise.
« Reply #6 on: 13/05/2003 21:33:08 »
Thank you good answer, althought it's a bit advance for me but I think I still understand it.

Tom
 

Offline Quantumcat

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Re: Lasers and noise.
« Reply #7 on: 14/05/2003 10:38:36 »
I remember studying lasers two years ago in physics ...

And Tom you asked if you can see laser light if it is intensified, and the answer is no, because light from something you're seeing has to enter your eyes. Laser light is in a perfect straight line, so none enters your eyes and you can't see it. You can see the dot on the thing you're aiming it at because the light is splayed off so it can enter your eyes. If you can see a laser beam it is because it is a crappy laser, and a lot of the light doesn't go in a straight line like it should, instead splays out immediately and that can enter your eyes. also sometimes the light will bounce of air particles and you can see the light that is bounced off.
 

Offline nilmot

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Re: Lasers and noise.
« Reply #8 on: 16/05/2003 20:16:10 »
So if people are getting laser treatment erm... for instant on their eyes or face won't it be hard to control and figure out how to do it because it has to be very precise or else someone's eyes are going to be seriously damaged?

Tom
 

Offline NakedScientist

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Re: Lasers and noise.
« Reply #9 on: 17/05/2003 00:19:25 »
Spot on ! Literally.

Lasers have a number of applications in the treatment of various eye conditions including diabetes, retinal detachment and laser correction of vision (see Dalya Rosner's article about LASIK - http://www.thenakedscientists.com/HTML/articles/article/dalyacolumn4.htm)

But when used in this way the laser beam is 'dispensed' in short bursts which delivers a highly focused shot at a specific part of the eye, lined up using a microscope.

In diabetic eye disease new blood vessels grow into the retina and threaten vision by bleeding. These vessels are removed by burning bits of them with a laser which also takes out a small segment of the retina, but since the aim is to spare the central area of vision, this is a worthwhile payoff because it generates only a minute decrement in peripheral vision.

If the opthalmologist missed with the laser and hit the wrong bit of the eye you could destroy someone's vision for ever, very easily. In fact a man did precisely that when he accidentally shone a laser into his eye from an indistrial machine. Whilst only low power it was sufficient to burn out the central visual area (the macula), which is the bit we need for reading and highly accurate vision, and now he has a huge black circle right in the middle of his vision and can only see what is going on around the edges.

TNS
 

Offline nilmot

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Re: Lasers and noise.
« Reply #10 on: 18/05/2003 20:27:20 »
Dear God that's vile!!! Poor man walking around with a black hole in his eye!![B)]

Tom
 

Offline lightarrow

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Re: Lasers and noise.
« Reply #11 on: 10/08/2006 18:04:11 »
quote:
Originally posted by Quantumcat

I remember studying lasers two years ago in physics ...

And Tom you asked if you can see laser light if it is intensified, and the answer is no, because light from something you're seeing has to enter your eyes. Laser light is in a perfect straight line, so none enters your eyes and you can't see it. You can see the dot on the thing you're aiming it at because the light is splayed off so it can enter your eyes. If you can see a laser beam it is because it is a crappy laser, and a lot of the light doesn't go in a straight line like it should, instead splays out immediately and that can enter your eyes. also sometimes the light will bounce of air particles and you can see the light that is bounced off.


Of course you were speaking about a laser in the void, or a low-intensity laser (like a laser pointer). Stronger laser beams can be seen well even in clean air: diffusion from air molecules. The same reason you can see all sky (blue) during the day, and not only direct light from the sun.
 

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Re: Lasers and noise.
« Reply #11 on: 10/08/2006 18:04:11 »

 

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