Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

There's a limit.

Very slight beam divergence is one, and for most applications, atmospheric
absorption and scattering.


--
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On 29/07/2013 20:59, Tim Watts wrote:
On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

There's a limit.

Very slight beam divergence is one, and for most applications, atmospheric
absorption and scattering.


Ok, thanks for that.

On Mon, 29 Jul 2013 20:47:24 +0100, bod wrote:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

Unless it absolutely parallel the beam will diverge (even if it
converges first) and matter in the path will scatter the beam. So in
practical terms a laser beam is not infinite but with very careful
collimation and could be very long.

--
Cheers
Dave.

On 29/07/2013 21:14, Dave Liquorice wrote:
On Mon, 29 Jul 2013 20:47:24 +0100, bod wrote:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

Unless it absolutely parallel the beam will diverge (even if it
converges first) and matter in the path will scatter the beam. So in
practical terms a laser beam is not infinite but with very careful
collimation and could be very long.

Gotcha, cheers.

On Mon, 29 Jul 2013 21:02:09 +0100, bod wrote:

On 29/07/2013 20:59, Tim Watts wrote:
On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:

Being as a laser maintains its power in its beam. Does anyone know if
a laser beam is infinite in how far the beam shines or is there a
limit?

There's a limit.

Very slight beam divergence is one, and for most applications,
atmospheric absorption and scattering.


Ok, thanks for that.

http://www.thenakedscientists.com/HT...question/2672/

--
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My posts (including this one) are my copyright and if @diy_forums on
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*lightning surge protection* - a w_tom conductor

On 29/07/2013 20:59, Tim Watts wrote:
On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

There's a limit.

Very slight beam divergence is one, and for most applications, atmospheric
absorption and scattering.


You could see examples of this in the demonstrations in Egypt where
people were shining red and green laser pointers at helicopters
resulting, IIRC, in beam diameters approaching a metre at helicopter
height. My guess is that most of this was divergence, rather than
scattering, associated with cheap laser pointer optics.

ISTR that US military experiments were getting something like metre
divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a
laser pointer with a 1 mm diode, the diffraction angle is of the order
of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military
laser has a 100 mm "objective", now the divergence is 1 in 200,000 or a
metre in 200 km.

On 29/07/2013 22:25, newshound wrote:
On 29/07/2013 20:59, Tim Watts wrote:
On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

There's a limit.

Very slight beam divergence is one, and for most applications,
atmospheric
absorption and scattering.


You could see examples of this in the demonstrations in Egypt where
people were shining red and green laser pointers at helicopters
resulting, IIRC, in beam diameters approaching a metre at helicopter
height. My guess is that most of this was divergence, rather than
scattering, associated with cheap laser pointer optics.

ISTR that US military experiments were getting something like metre
divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a
laser pointer with a 1 mm diode, the diffraction angle is of the order
of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military
laser has a 100 mm "objective", now the divergence is 1 in 200,000 or a
metre in 200 km.

They estimate that if you project a laser spot on the moon, its about
40m wide by the time it gets there...

--
Cheers,

John.

/================================================== ===============\
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\================================================= ================/

On 29/07/2013 23:06, John Rumm wrote:
On 29/07/2013 22:25, newshound wrote:
On 29/07/2013 20:59, Tim Watts wrote:
On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

There's a limit.

Very slight beam divergence is one, and for most applications,
atmospheric
absorption and scattering.


You could see examples of this in the demonstrations in Egypt where
people were shining red and green laser pointers at helicopters
resulting, IIRC, in beam diameters approaching a metre at helicopter
height. My guess is that most of this was divergence, rather than
scattering, associated with cheap laser pointer optics.

ISTR that US military experiments were getting something like metre
divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a
laser pointer with a 1 mm diode, the diffraction angle is of the order
of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military
laser has a 100 mm "objective", now the divergence is 1 in 200,000 or a
metre in 200 km.

They estimate that if you project a laser spot on the moon, its about
40m wide by the time it gets there...


Same sum says that means a 5 metre lens or mirror. Which, by chance, is
the size of the Hale telescope on Mount Palomar.

On 29/07/13 20:47, bod wrote:
Being as a laser maintains its power in its beam. Does anyone know if
a laser beam is infinite in how far the beam shines or is there a limit?

It doesn't maintain it THAT well


--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

On 29/07/13 21:14, Dave Liquorice wrote:
On Mon, 29 Jul 2013 20:47:24 +0100, bod wrote:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?
Unless it absolutely parallel the beam will diverge (even if it
converges first) and matter in the path will scatter the beam. So in
practical terms a laser beam is not infinite but with very careful
collimation and could be very long.

ISTR we (humans) bounced one off the moon and it was detectable with an
optical telescope


--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

On 29/07/2013 21:37, Bob Eager wrote:
On Mon, 29 Jul 2013 21:02:09 +0100, bod wrote:

On 29/07/2013 20:59, Tim Watts wrote:
On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:

Being as a laser maintains its power in its beam. Does anyone know if
a laser beam is infinite in how far the beam shines or is there a
limit?

There's a limit.

Very slight beam divergence is one, and for most applications,
atmospheric absorption and scattering.


Ok, thanks for that.

http://www.thenakedscientists.com/HT...question/2672/

Some useful info in that link, thanks.

Not quite sure what you mean. Its not really any different to any other
light except its coherent much like a radio wave, and can be better focussed
of course, but it still diverges and is affected by turbulenceand impurities
in the medium it is going through.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"bod" wrote in message
...
Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

On 30/07/2013 09:16, Brian Gaff wrote:
Not quite sure what you mean. Its not really any different to any other
light except its coherent much like a radio wave, and can be better focussed
of course, but it still diverges and is affected by turbulenceand impurities
in the medium it is going through.

Brian

I asked the question after hearing a quote from a presenter on a
documentary about lasers. He stated that "a laser does not lose any
power over distance".

On Tuesday 30 July 2013 09:24 bod wrote in uk.d-i-y:

On 30/07/2013 09:16, Brian Gaff wrote:
Not quite sure what you mean. Its not really any different to any other
light except its coherent much like a radio wave, and can be better
focussed of course, but it still diverges and is affected by
turbulenceand impurities in the medium it is going through.

Brian

I asked the question after hearing a quote from a presenter on a
documentary about lasers. He stated that "a laser does not lose any
power over distance".

...."in a vacuum". But that's true of any light.

And it will still lose power per area as the beam diverges.

--
Tim Watts Personal Blog: http://squiddy.blog.dionic.net/

http://www.sensorly.com/ Crowd mapping of 2G/3G/4G mobile signal coverage

Reading this on the web? See:
http://wiki.diyfaq.org.uk/index.php?title=Usenet

On 30/07/2013 09:35, Tim Watts wrote:
On Tuesday 30 July 2013 09:24 bod wrote in uk.d-i-y:

On 30/07/2013 09:16, Brian Gaff wrote:
Not quite sure what you mean. Its not really any different to any other
light except its coherent much like a radio wave, and can be better
focussed of course, but it still diverges and is affected by
turbulenceand impurities in the medium it is going through.

Brian

I asked the question after hearing a quote from a presenter on a
documentary about lasers. He stated that "a laser does not lose any
power over distance".

..."in a vacuum". But that's true of any light.

And it will still lose power per area as the beam diverges.

Ah! the presenter didn't elaborate about divergence etc. He just made
that sweeping statement, hence, my question.

On Tuesday, 30 July 2013 07:38:20 UTC+1, bod wrote:
On 29/07/2013 21:37, Bob Eager wrote:

On Mon, 29 Jul 2013 21:02:09 +0100, bod wrote:



On 29/07/2013 20:59, Tim Watts wrote:

On Monday 29 July 2013 20:47 bod wrote in uk.d-i-y:



Being as a laser maintains its power in its beam. Does anyone know if

a laser beam is infinite in how far the beam shines or is there a

limit?



There's a limit.



Very slight beam divergence is one, and for most applications,

atmospheric absorption and scattering.





Ok, thanks for that.



http://www.thenakedscientists.com/HT...question/2672/



Some useful info in that link, thanks.

http://en.wikipedia.org/wiki/Lunar_L...ing_experiment

"At the Moon's surface, the beam is only about 6.5 kilometers (four miles) wide[6"

On Tue, 30 Jul 2013 00:19:22 +0100, The Natural Philosopher wrote:

So in practical terms a laser beam is not infinite but with very
careful collimation and could be very long.

ISTR we (humans) bounced one off the moon and it was detectable with an
optical telescope

The moon isn't very far away. At least one of the Apollo missions
left a retro-reflector on the surface fairly sure that they regularly
fire a laser at that and look for the reflection to measure the
distance between earth and moon to a cm or three. B-)

--
Cheers
Dave.

http://www.thenakedscientists.com/HT...question/2672/



Some useful info in that link, thanks.

http://en.wikipedia.org/wiki/Lunar_L...ing_experiment

"At the Moon's surface, the beam is only about 6.5 kilometers (four miles) wide[6"

Cheers.

On Mon, 29 Jul 2013 22:25:37 +0100, newshound wrote:

ISTR that US military experiments were getting something like metre
divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a
laser pointer with a 1 mm diode, the diffraction angle is of the order
of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military
laser has a 100 mm "objective", now the divergence is 1 in 200,000 or a
metre in 200 km.

Care to explain this "diffraction limit"?

I can't see a (theoretical) reason why you can't construct an optical
system that will produce a parallel beam. If there is some physical
reason to do with photons that means that optics that should create a
parallel beam don't you just put a "correction" into the optics to
take that into account.

--
Cheers
Dave.

Dave Liquorice wrote:
On Mon, 29 Jul 2013 22:25:37 +0100, newshound wrote:

ISTR that US military experiments were getting something like metre
divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a
laser pointer with a 1 mm diode, the diffraction angle is of the order
of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military
laser has a 100 mm "objective", now the divergence is 1 in 200,000 or a
metre in 200 km.

Care to explain this "diffraction limit"?

I can't see a (theoretical) reason why you can't construct an optical
system that will produce a parallel beam. If there is some physical
reason to do with photons that means that optics that should create a
parallel beam don't you just put a "correction" into the optics to
take that into account.

I'm not completely au fait with the maths, but basically, the smaller a
lens or mirror is, the less sharply it will focus a beam. Using smaller
apertures on a camera lens will increase apparent sharpness of the image
until you get to the size where the rays start diffracting, at which
point detail starts getting more blurred.

Going the other way, the bigger the searchlight lens/mirror combination,
the more parallel you can make the light rays in the beam. It also
affects radio transmissions and sound recording using parabolic
reflectors, where the bigger the dish, the smaller the beam angle.

If you want the gory details, then search for diffraction limit on Google.

--
Tciao for Now!

John.

Dave Liquorice wrote:
On Tue, 30 Jul 2013 00:19:22 +0100, The Natural Philosopher wrote:

So in practical terms a laser beam is not infinite but with very
careful collimation and could be very long.
ISTR we (humans) bounced one off the moon and it was detectable with an
optical telescope

The moon isn't very far away. At least one of the Apollo missions
left a retro-reflector on the surface fairly sure that they regularly
fire a laser at that and look for the reflection to measure the
distance between earth and moon to a cm or three. B-)

5mm according to this, with +-3mm expected shortly:-

http://www.fesg.bv.tum.de/91872-bD1l...chung~llr.html

--
Tciao for Now!

John.

On 30/07/13 10:49, Dave Liquorice wrote:
On Mon, 29 Jul 2013 22:25:37 +0100, newshound wrote:

ISTR that US military experiments were getting something like metre
divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a
laser pointer with a 1 mm diode, the diffraction angle is of the order
of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military
laser has a 100 mm "objective", now the divergence is 1 in 200,000 or a
metre in 200 km.
Care to explain this "diffraction limit"?

I can't see a (theoretical) reason why you can't construct an optical
system that will produce a parallel beam. If there is some physical
reason to do with photons that means that optics that should create a
parallel beam don't you just put a "correction" into the optics to
take that into account.

I THINK there is some fairly deep quantum level physics to explain why
light in the company of other light doesn't travel in straight lines.

In any case there is some kind of diffraction limit.

But ten minutes interesting googling has not revealed an exact answer to
your question.

--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

On 30/07/13 18:31, Tim Streater wrote:
In article ,
The Natural Philosopher wrote:

On 30/07/13 10:49, Dave Liquorice wrote:
On Mon, 29 Jul 2013 22:25:37 +0100, newshound wrote:

ISTR that US military experiments were getting something like metre
divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a
laser pointer with a 1 mm diode, the diffraction angle is of the
order
of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military
laser has a 100 mm "objective", now the divergence is 1 in 200,000
or a
metre in 200 km.
Care to explain this "diffraction limit"?

I can't see a (theoretical) reason why you can't construct an optical
system that will produce a parallel beam. If there is some physical
reason to do with photons that means that optics that should create a
parallel beam don't you just put a "correction" into the optics to
take that into account.

I THINK there is some fairly deep quantum level physics to explain
why light in the company of other light doesn't travel in straight
lines.

Light is a wave and will spread out beyond the straight-line confines
of the exit diameter of the device. The sharpness of the image in your
camera is similarly limited. The more you stop down the lens, the
worse it gets.

yeah, I think around f8 is optimal for MOST lenses.

somewhere between lens spherical distortion and diffraction :-)

--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

On 30/07/2013 10:49, Dave Liquorice wrote:
Care to explain this "diffraction limit"?

I can't see a (theoretical) reason why you can't construct an optical
system that will produce a parallel beam. If there is some physical
reason to do with photons that means that optics that should create a
parallel beam don't you just put a "correction" into the optics to
take that into account.

It's quantum innit. Once you've got your head properly around
wave-particle duality and all that stuff it becomes clear.

This is not I might add until post-graduate physics. Undergrads are well
known for realising how little physics they really understand. I
understand Newton, and that's good enough for most purposes.

Andy

On 30/07/13 22:01, Tim Streater wrote:
In article ,
Vir Campestris wrote:

On 30/07/2013 10:49, Dave Liquorice wrote:
Care to explain this "diffraction limit"?

I can't see a (theoretical) reason why you can't construct an optical
system that will produce a parallel beam. If there is some physical
reason to do with photons that means that optics that should create a
parallel beam don't you just put a "correction" into the optics to
take that into account.

It's quantum innit. Once you've got your head properly around
wave-particle duality and all that stuff it becomes clear.

This is not I might add until post-graduate physics. Undergrads are
well known for realising how little physics they really understand. I
understand Newton, and that's good enough for most purposes.

Certainly for landing a spacecraft anywhere in the Solar System. But
if you want to design computer chips, you need quantum physics. If you
want to use those chips to make a SatNav, you need to take relativity
into account.

Not QUITE true. You don't need to understand HOW a transistor works to
design circuits USING them.
Designing transistors...is a different matter.




--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.

On 30/07/2013 22:01, Tim Streater wrote:

Certainly for landing a spacecraft anywhere in the Solar System. But if
you want to design computer chips, you need quantum physics.

I think you mean design semiconductor processes and transistors, etc,
you don't need quantum mechanics to design computer chips.
You will probably need to understand heat flow and a few other things.

If you want
to use those chips to make a SatNav, you need to take relativity into
account.


Or just use the formulae provided.

On 29/07/2013 21:14, Dave Liquorice wrote:
On Mon, 29 Jul 2013 20:47:24 +0100, bod wrote:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

Unless it absolutely parallel the beam will diverge (even if it
converges first) and matter in the path will scatter the beam. So in
practical terms a laser beam is not infinite but with very careful
collimation and could be very long.



They can bounce Lasers off the moon for example.
There were reflector arrays left on moon by Apollo & Russian missions
specifically for this test. (researching relativity etc.)
They used a very tightly focussed commercial Laser that output light
pluses that had only one arcsecond (1/3600th of a degree) divergence.
This still resulted in a spot to 1.8 kilometers wide on the moon.


http://what-if.xkcd.com/13/

On 01/08/2013 12:07, Huge wrote:


http://what-if.xkcd.com/13/

I hate you. Every time someone posts a "what-if" xkcd link, I then spend
an hour reading them and all the associated links.




:-)

In article , Huge
scribeth thus
On 2013-08-01, Rick Hughes wrote:
On 29/07/2013 21:14, Dave Liquorice wrote:
On Mon, 29 Jul 2013 20:47:24 +0100, bod wrote:

Being as a laser maintains its power in its beam. Does anyone know if a
laser beam is infinite in how far the beam shines or is there a limit?

Unless it absolutely parallel the beam will diverge (even if it
converges first) and matter in the path will scatter the beam. So in
practical terms a laser beam is not infinite but with very careful
collimation and could be very long.



They can bounce Lasers off the moon for example.
There were reflector arrays left on moon by Apollo & Russian missions
specifically for this test. (researching relativity etc.)
They used a very tightly focussed commercial Laser that output light
pluses that had only one arcsecond (1/3600th of a degree) divergence.
This still resulted in a spot to 1.8 kilometers wide on the moon.


http://what-if.xkcd.com/13/

I hate you. Every time someone posts a "what-if" xkcd link, I then spend
an hour reading them and all the associated links.



+ 1 or several;!...
--
Tony Sayer

tony sayer wrote:
In article , Huge
scribeth thus
On 2013-08-01, Rick Hughes wrote:

http://what-if.xkcd.com/13/
I hate you. Every time someone posts a "what-if" xkcd link, I then spend
an hour reading them and all the associated links.



+ 1 or several;!...

http://www.xkcd.com/1190/

Is still changing.
--
Tciao for Now!

John.

On Tue, 30 Jul 2013 18:31:40 +0100, Tim Streater wrote:

I can't see a (theoretical) reason why you can't construct an
optical
system that will produce a parallel beam. If there is some
physical
reason to do with photons that means that optics that should
create a
parallel beam don't you just put a "correction" into the optics
to
take that into account.

I THINK there is some fairly deep quantum level physics to explain
why
light in the company of other light doesn't travel in straight
lines.

Light is a wave and will spread out beyond the straight-line confines of
the exit diameter of the device.

Ha! Quantum stuff and wave-particle duality... I was in "particle"
mode.

--
Cheers
Dave.

"Dave Liquorice" writes:

On Tue, 30 Jul 2013 18:31:40 +0100, Tim Streater wrote:

I can't see a (theoretical) reason why you can't construct an
optical
system that will produce a parallel beam. If there is some
physical
reason to do with photons that means that optics that should
create a
parallel beam don't you just put a "correction" into the optics
to
take that into account.

I THINK there is some fairly deep quantum level physics to explain
why
light in the company of other light doesn't travel in straight
lines.

Light is a wave and will spread out beyond the straight-line confines of
the exit diameter of the device.

Ha! Quantum stuff and wave-particle duality... I was in "particle"
mode.

Its worth considering this argument: according to the
uncertainty principle, the better you know the momentum of
something, the less well you can know its position and vice
versa. With light you know the speed and mass, and the smaller
the hole it goes through, the better you know the position, so
the only thing left to be ignorant about is the direction.

Its so long that I learnt any of this stuff that I may be
talking rubbish, though.

--
Jón Fairbairn
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2012-10-07)

On 30/07/2013 10:49, Dave Liquorice wrote:
On Mon, 29 Jul 2013 22:25:37 +0100, newshound wrote:

ISTR that US military experiments were getting something like metre
divergence at ~ 100 miles. (The "diffraction limit" sum is trivial).

In fact let's do the sum. Assume a wavelength of half a micron and a
laser pointer with a 1 mm diode, the diffraction angle is of the order
of 1 in 2000 or a metre diameter at 2 kilometers. Suppose a military
laser has a 100 mm "objective", now the divergence is 1 in 200,000 or a
metre in 200 km.

Care to explain this "diffraction limit"?

I can't see a (theoretical) reason why you can't construct an optical
system that will produce a parallel beam. If there is some physical
reason to do with photons that means that optics that should create a
parallel beam don't you just put a "correction" into the optics to
take that into account.

Even with an optical system you are going to get some diffraction at the
edges of the beam where you collimate it.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Tim Streater wrote:
Look he

http://en.wikipedia.org/wiki/Diffraction_limited

An optical system will be limited by diffraction basically because light
(like all EM) is a wave in this context (and a particle in others, such
as the photo-electric effect).

Nothing to be done about it.

Sure there is - just use a Bessel beam. The fact that that beam
profile is also infinitely wide is surely no problem :-)

#Paul