Laser Pens

[drlove]

I happened to be out this evening walking past a bus stop. There were some girls standing there playing with a laser pen. Since I wasn't expecting it, I accidently looked right at it as they were waving it around before I realized what it was.

Exactly how dangerous are they?? I heard they can blind you and/or cause serious damage if you look at them too long.

When will people realize these are not toys??

 

[WhOiSyOdAdDy?]

I once was at a bus stop.. and one was pointed at my chest, and I almost jumped into traffic because I thought that there was a sniper targetting me.

 

[Diode]

Oh Jeez, guys, please RELAX! a POWERFULL, laser, is in the order of 1mW ( milli-watt, or millionth of a watt)

While, laser light is somewhat unique, the absorbed power at a location is still influencd by what is called the INVERSE SQUARE LAW.

Which means, the absorbed power at any individual point falls off by the SQARE of the distance.

Please don't panic! We are not talking about some HUGE amount of power! Think about it, how much heat can you get from a AAA battery?

I thought perhaps, TERB members, were a bit less wimpy!

Anyway, watch out for those mean school girls with their pointers, they might just try and teach you somthing!

 

[aptenodytes]

The inverse square law applies to light as well, generally, but a laser is a bit of a different animal since it's a coherent beam. All the little photons are actually travelling parallel to each other, unlike say the light from a light bulb. Inverse square applies in the non-coherent case.

 

[Diode]

Holy crap, Winston. WRONG! Light unfortunatly, can be defined TWO WAYS! The so called 1)partical theory/ or2) wave theory.

The partical theory, is effective in some defintions, look up Quanta, or PHOTONS!

The wave theory, is nice in this deffinition. Think of any electro-magnetic wave, it MUST by defintion move from its FOCUS in a determined pattern. This patern is DIVERGENT, from the focus.

While at a measurable distance, fom the focus, a quanity of energy my be defined as NON-Divergent, energy is still traveling in all directions from the focus.

The key to a LASER, is that the focus is approaching a zero 'size', or a "pin-point"

Therfore, while divergence, occurs, the appearence, is very small.

The so-called area product, remains the same, BUT the absorbed dose at a particular point must fall of as the square of the distance.

Your point about my eyes is EXACTLY right!

Not from arms length, but across thy street? NO PROBLEM.

With all due respect Winston, "Do not try and teach your grandmother how to suck eggs"

Oh, all light, laser,spectrum, x-ray, gamma rays, ect travell to infinity in a vacuum. But they all travel in divergent paths!

Regards
DIODE

 

[KBear]

Diode, that makes no sense what so ever.... hmm, going to get my physics book, and my dictionary.

"energy is still traveling in all directions from the focus." kind of defeats the purpose of using a laser.

 

[xarir]

KBear - it actually does make sense. Look up how a laser works; you'll see that it's basically light bouncing around between 2 semi-transparent mirrors. A very small portion of light "escapes" which is is the coherent light beam. At the focii of the semi-transparent mirrors, there's a lot of energy (quanta) that are moving in all directions.

Anyhow to drlove's intial query, a laser pointer is not necessarily dangerous. You get more energy absorption / dissipation from a camera flash. While it's obviously not recommended to look straight into any kind of laser, you're not going to go blind if you happen to get lased from a pointer. It might be annoying, but it's not really dangerous either. At least, it's no more dangerous than a powerful flashlight, or a camera flash, or xenon lights on a car.

 

[KBear]

It makes no sense to me that the energy from a laser would drop off by the square of the distance. ie, you would be significantly safer to be hit in the eye from a greater distance from true laser light. I would really like to read more about that, is there an online article?

 

[xarir]

It makes no sense to me that the energy from a laser would drop off by the square of the distance. ie, you would be significantly safer to be hit in the eye from a greater distance from true laser light. I would really like to read more about that, is there an online article?

This pdf examines a quantum system driven by an external field (laser) which is coupled to a reservoir (environment). The math is advanced, but basically it demonstrates from both a classical and quantum perspective that energy dissipation increases over distance as a laser beam interacts with the environment.

http://arxiv.org/PS_cache/physics/pdf/9911/9911034.pdf

 

[papasmerf]

Does not the ammout of moisture in the air affect ti light refraction, and disipate more light in heavey air???????

 

[Diode]

Xair is right. Sorry for the incoheance last night, but too many beers.

Now, any type of light, can be considerd to contain Mostly cohearent rays at a infinite distance.

Again lasers are unique, but as Xair stated, a laser operates between two mirriors, one being totaly internally reflective (close to 99.999% of the energy is reflected) and another mirror, which is approx 98-99% reflective.

This bouncing action, causes the cohearent rays, to act the same as if the focus was infinitly far away.

Now, what we are talking about with laser pointers, are actually laser DIODES ( funny that's my handle),

A diode is simpily, a device that allows current to pass in one direction only.

Some diodes, LED's for example emit light.

A laser diode is specifically designed, to emit a type of light that is close, to, but not the same as true laser light.

If you don't belive me that the light is not divergent, try this test.

Remove the lens from pointer, point at white paper at 6 inches, circle the "dot", now move white paper 10, 20 feet away, repeat measurement, is dot bigger? Yes! Why, divergent rays.


Papasmerf: Good question, refraction and absorbtion are different animals!

Refraction is a great demonstrater of the Particle nature of light!!!

Absorbtion, is a demonstrater, of the wave nature of light.

Physics is really cool!

 

[KBear]

Diode wrote >>>"Remove the lens from pointer, point at white paper at 6 inches, circle the "dot", now move white paper 10, 20 feet away, repeat measurement, is dot bigger? Yes! Why, divergent rays."

Yes, well of course, this is not a laser with the lens removed, it is a flashlight. The energy hitting a specific sized area at one distance will be higher then the energy hitting the same sized object at a further distance, because as the light beam moves further away it expands.

But the light from a laser does not expand over distance, so why does the energy of the laser beam drop off so dramatically as you pointed out in your original post? There will certainly be minor losses as the beam is scattered by water droplets, etc in the air, but this does not account for the losses you described in your post >> Diode wrote>>> “Which means, the absorbed power at any individual point falls off by the SQARE of the distance.”

Xarir, the article you found has nothing to do with Diode’s statement that “absorbed power at any individual point falls off by the SQARE of the distance”. ”. Which would make it significantly safer to be hit with a laser light from across the street, then from arms length. However, this is false!!

Please, no more explanations of how the laser light is produced.

 

[HowardHughes]

"lasers"

I have in fact worked with lasers in the physics lab at U of T.

The short of the story is that power emitted from a "laser pen" might cause a slight distortion in someone's eye (due to the retina adjusting) - HOWEVER - the power emitted would not blind someone - nor burn paper, that kind of thing.

Now...if we could just attach them to ill-tempered sea bass...

 

[HowardHughes]

and...

Not to mention - you really can't generate enough power from a AAA battery to do damage...unless you've figured out cold-fusion...

 

[KBear]

would agree that a flash of light from these laser pointers would not likely do any real damage to your eye, regardless of the distance.

 

[jwmorrice]

Re: and...

Not to mention - you really can't generate enough power from a AAA battery to do damage...unless you've figured out cold-fusion...

Not so. You don't take into account the focusing action of the eye. See below from Princeton University's Environmental Health and Safety page:

"The major danger of laser light is hazards from beams entering the eye. The eye is the organ most sensitive to light. Just as a magnifying glass can be used to focus the sun and burn wood, the lens in the human eye focuses the laser beam into a tiny spot than can burn the retina. A laser beam with low divergence entering the eye can be focused down to an area 10 to 20 microns in diameter.

The laws of thermodynamics do not limit the power of lasers. The second law states that the temperature of a surface heated by a beam from a thermal source of radiation cannot exceed the temperature of the source beam. The laser is a non-thermal source and is able to generate temperatures far greater than it's own. A 30 mW laser operating at room temperature is capable of producing enough energy (when focused) to instantly burn through paper.

Per the law of the conservation of energy, the energy density (measure of energy per unit of area) of the laser beam increases as the spot size decreases. This means that the energy of a laser beam can be intensified up to 100,000 times by the focusing action of the eye. If the irradiance entering the eye is 1 mW/cm2, the irradiance at the retina will be 100 W/cm2. Thus, even a low power laser in the milliwatt range can cause a burn if focused directly onto the retina.

NEVER point a laser at someone's eyes no matter how low the power of the laser."

jwm

 

[HowardHughes]

okay

that is true and all - I would also say the same with a magnifying glass - however - unless you stand there, point the laser 6 inches from your eye - don't blink - and stare into it for a length of time -yeah - it will cause damage. Those fellows at Princeton better give out the same warning about convex glass surfaces too then.

However, getting a flash of it is not going to send you to the CNIB.

I have managed to get burned by a laser - it isn't fun - it was pretty high intensity - but it did very little damage - that was in a controlled physics lab.

A pen light on a street corner is not a weapon of mass destruction.

Just my humble, reclusive opinion.

 

[Taboo]

Granny always told me never to stare at any kind of light directly because it will harm your eyes.

 

[xarir]

Re: Re: and...

A 30 mW laser operating at room temperature is capable of producing enough energy (when focused) to instantly burn through paper.

That's probably true. But a 30mW laser is a BEAST! The lasers in pointers, grocery store scanners etc are less than 0.5mW. (Probably around 0.25mW or even less than that.)

A 30mW laser is industrial strength stuff and generally is not available for sale to the public. Even most medical lasers wouldn't be this powerful.

 

[CyberGoth]

hey, I'm supposed to be the paranoid one here.

ok, now THAT is paranoid.

laser pointers are useful little tools. they make great cat toys. and laser sights dont look quite like that when you're on the recieving end.

I found that when I've been laserflashed by accident, that mirrorshade sunglasses do reduce but to not eliminate the effects and yes, intense sustained lasering of an eyeball will probably cause damage.

snipers? where do you think you are? the U.S.?

I once was at a bus stop.. and one was pointed at my chest, and I almost jumped into traffic because I thought that there was a sniper targetting me.