(probably a bit OT) Laser question

[John Williamson]

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.

[John Williamson]

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

[The Natural Philosopher]

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.

[Tim Streater]

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.

[Tim Streater]

The light is always diffracting, as the aperture gets smaller the effect gets bigger.

[The Natural Philosopher]

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

somewhere between lens spherical distortion and diffraction :-)

[Vir Campestris]

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

[Tim Streater]

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.

[The Natural Philosopher]

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

[dennis]

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.

Or just use the formulae provided.

[Rick Hughes]

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.

[Huge]

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

[Rick Hughes]

:-)

[tony sayer]

• 1 or several;!...

[John Williamson]

Is still changing.

[Dave Liquorice]

physical

create a

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

[Jon Fairbairn]

It?s 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.

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

[John Rumm]

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

[Tim Streater]

Look here:

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.

[news13k]

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

#Paul