Beam brightness

[laserLips]

Sorry if this has been asked before, or is obviouse, but why do beams comming towards you look brigher than beams going away from you?
Graham

[buffo]

Because Rayleigh and Mie scattering both tend to deflect the beam path only very slightly. So if it's already headed towards you, the beam doesn't need to be scattered very far off it's original path for some of the light to enter your eye. But if the beam is going away from you, the light needs to make a 180 degree turn to get to your eye, and that's not the way scattering works. (Some does still reach your eye, of course, but it requires *many* successive scattering events in order to do so, thus the intensity is lower.)

Note that if you were using REFLECTIVE particles instead of a fog or haze, then the beam would look brighter when it's headed away from you, since reflection *does* cause a large change in the beam path.

Adam

[laserLips]

Thanks Adam ;-)

[Jem]

Hi Graham

We've not heard from you in a while, are you o.k?

Nice to see you back again.

Jem

[laserLips]

Hi Jem, Don't get much opportunity to play with my laser so it's been put into a cupboard and left to slumber.
Shame as it was very foggy in London last night so I could have got it out but. Been working hard at work and trying to develop some apps for a phone but all failed miserably. Also been trying to deal with my Tinnitus and hearing loss by building a website. http://t4tinnitus.com in case any of you have Tinnitus and/or hearing loss.
I will get my laser out when I get my own place. I just don’t get the full enjoyment out of it in my bedroom. I house share you see.
I do still get my laser fix every day though. I live in Greenwich and they have the Observatory laser on every night, which follows the meridian line. I'd love to know how powerful it is.
What's been happening round here then?
Graham

[Jem]

What's been happening round here then?

, Probably best not to ask, I think blissful ignorance is the way to go

Sorry to hear about your tinnitus, my Dad suffered badly from the condition

Take care

Jem

[smogthemog]

Aligning the Greenwich laser

From time to time, we have to check and adjust the alignment of the Meridian laser. Rather conveniently, back in 1824, the sixth Astronomer Royal John Pond had an Obelisk erected at Pole Hill some 11 miles from the Observatory on the then Greenwich Meridian.
Its purpose was to provide an additional alignment check for the Transit Telescope that was then in use. Having initially adjusted the telescope to the Meridian from observations of the circumpolar stars, Pond lowered it to the horizon to determine exactly where the obelisk should be built. By this means, he was able to position it with an accuracy of about ± four inches. The Meridian defined by this particular telescope is known as the Bradley Meridian and is the Meridian used (for historic reasons) by the Ordnance Survey (OS) on its maps. It predates the current Greenwich Meridian, which runs about six metres further to the east at Greenwich and is defined by the Airy Transit Circle (ATC) of 1850. An OS trig point was erected adjacent to the Pond Obelisk and very close to present Meridian in the 1930s, and this is where I will be heading later this month.
From there I will be able to talk with my colleagues back at Greenwich and tell them in which direction (east or west) they need to move the beam. The beam will also be adjusted vertically so that it just grazes or clears the tops of the trees, thereby ensuring maximum visibility for those places further to the north. So if early one winter's evening, you think that you can see the beam moving around a bit, your eyes probably won't be deceiving you ... it will simply be us doing one of the regular adjustments that are sometimes needed!
For those who want a bit of info about the laser itself, it's a Millennia VS Diode-Pumped, cw Visible Laser. It has a wavelength of 532 nm and an output power > 5W. The laser unit is located beneath the Airy Transit Circle and 'fired' along the Meridian from above via a fibre optic as can be seen in the picture above.

[Jem]

... it will simply be us doing one of the regular adjustments that are sometimes needed!

Hey Jim,

Are you telling us that you're involved in adjusting and aligning that green beastie?... Cool

Jem

[Laser Ben]

Obviously this post wasn't needed...

[Doc]

I don't know what the hell he is saying.

The longitudinal meridian, along with sextants for calculating latitude and clocks that could keep time at sea enabled the British fleet to navigate and map the world.

It is the datum for every map of the Earth and seperates the eastern and western hemispheres.