Laser Question

When a LASER is switched off, does the entire beam cease to exist or is there a position far from the source where it still exist for a short time?

 

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Even if laser switch-off was instantaneous, obviously a finite length pulsed beam has been generated. Say it was switched on for some duration delta t, the pulse length is
L = c*delta t. And inversely, duration is easily found if knowing the pulse length.
But I'm puzzled - are you in fact one pair of twins, each having signed up here? Because as pointed out elsewhere, you have an identical birth date to (current )member Willem.

 

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Lasers project photons. Photons travel at the speed of light.
If you switched off a laser that was pointed at a reflector on the Moon, it would take 2.6 before you saw the reflected light stop.

 

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I re-registered at Google and this forum's webpage must have contacted Google and re-registered me.

 

What happens to the photons on the edge, where the light got cut off? Do they spread again in the opposite direction as the laser light were going.

 

The laser is producing light by exciting electrons. It's not like a door opening and closing on a lightbulb.
When you switch off the laser, no more photons are produced or emitted.

A loose analogy might be to a machine gun firing bullets. The bullets travel from source to target as long as the trigger is pulled. When the trigger is released, no more bullets are fired.
It's not like extra bullets are flying around inside the gun, waiting to go down the barrel.

 

Sounds bizarre but given you are still here and not banned as sockpuppeteer, evidently admin agrees with that account. Interesting and maybe a bit scary.
With me, it's just plain manners to respond to answers that are non-frivolous. You have not responded to my #2 here (other than to clear up above extraneous issue). Nor to my #24 (#25 now resolved) in Speed of Light thread: http://www.sciforums.com/threads/speed-of-light.161944/page-2

 

The only thing you asked in #2 was an off-topic and personal digression that might have gotten him banned.
With me, it's just plain manners to ... not do that.

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Are you for real? I didn't just ask a question in #2 - or did that much escape notice? And that question was no different in character than the comment made here:
http://www.sciforums.com/threads/what-is-energy.161789/page-4#post-3579693
In fact, it was that post that inspired me to check the relevant details for Willem and talanun46.
Given you made no negative comment to above linked post, where is your consistency? My question was perfectly reasonable given the strange coincidence. The notion admin would ban Willem on the strength of merely that much from me is absurd.

Since you like to give some, even when ill-founded, be prepared to get some back. For not the first, I noticed you going cap-in-hand to PhysicsForums, get your arse kicked in the process, then after getting your confusion straightened out there, come back here and, without any acknowledgement of having your own errors being corrected elsewhere, play stern instructor of babes on those very same issues, typically with River:
https://www.physicsforums.com/threads/e-mc-2-where-did-the-1-2-go.971390/
http://www.sciforums.com/threads/bb...e-is-more-complete.161641/page-9#post-3575256

 

I can't immediately see where Dave got his "arse kicked" on that forum. He seems to have gone there for help with a question he had - and got it. If I want to set someone straight on a topic and I am not quite sure of myself, I go and check it somewhere first, with the object of avoiding talking out of my arse - whether or not someone may think it is being kicked.

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Sure and all that is fine per se, all of us can learn from others and that wasn't my beef with Dave. Which was, if you get correction elsewhere, have enough humility to admit it here, before going on to in turn harshly admonish who I guess passes here for the (indispensable) 'village idiot' on that same topic. Yes said 'VI' is stubborn, but why keep presssing once that is obvious? There's a line between genuinely useful dialog and sport for sports sake. I may have been too harsh myself in saying 'got arse kicked', but one or two posts in that PF thread come close enough imo. imho.

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I did reply: Google's fault.

So the edge of that beam does not look like a cut off rod?

 

I have nothing to do with that post.

 

The question about his identity might have gotten a pass, until you essentially admonished him for not answering it, when it was apparent he didn't want to.
I thought the irony of you mentioning "good manners" was worth highlighting.

Can we get back on topic please?

 

What mental model do you have of photons being emitted by a laser?

Again, consider the machine gun bullets analogy. Do bullets that don't get fired have a flattened tip because a door in the muzzle closed on them?

 

Sort of, yes. If you could visualize the photons it would look like a "rod" of photons traveling through space.

 

My interpretation of willem's "cut off rod" was at the muzzle of the laser. i.e. if you stop the laser, the photons in the laser would have a blunt front.

 

That issue seems to have been sorted out now. Still seems bizarre - like bots have a say in how this site works!

Owing to how a laser is constructed, it's never possible to instantaneously turn the beam on or off. So the leading and trailing edges of the finite duration pulse will not be sharp and of uniform intensity like a cut off rod. Also, a Fourier analysis will show there will be a significant spread in frequencies in those regions - considerably more than in the middle portion. As mentioned by others, the pulse simply propagates through space as a whole with speed c.
Finally, because the beam will have a finite angular spread (not an exact pencil beam), it spreads laterally as it propagates. A higher quality laser minimizes but cannot eliminate such spreading.

 

Sorry for any confusion created. I got a bit hot under the collar and let fly. Over and done now.