Why does the Greenwich laser seem to suddenly change direction?

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David Overton

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David Overton  asked the Naked Scientists:
Hi Chris,

Listening to your interesting programme last Sunday, reminded me of a strange phenomenon I've often observed concerning the Greenwich Millennium laser beam which shines directly over my house which just happens to be right on zero longitude.

If I walk just a few paces to the left or right of the beam it will suddenly change direction, is this because it is coherent light and there is hardly any scatter from side to side?

I get the same effect whilst driving under it as far away as Hertfordshire where it can still be clearly seen on a clear night . The effect seems to be more pronounced than  walking under a large structure like say the Humber suspension bridge.  At least I know I can always find my way home!

I wonder if anybody has any comments on this.

Great programme,
Best wishes
Dave Overton.

What do you think?
« Last Edit: 22/07/2010 18:30:02 by _system »


Offline LeeE

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How far from the Greenwich Observatory is your house?

The entire Humber bridge is a little under a mile and a half long, with the longest span being less than a mile.  If you live further away from the observatory than this then the laser beam will effectively be a larger/longer structure than the Humber bridge.
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Offline Soul Surfer

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What you are talking about is a perspective effect caused by the curvature of the earth's surface.  the laser beam is as you say straight and a fine line ands is probably exactly horizontal or slightly pointing upwards to avoid people looking directly into the beam at ground level.  the curvature of the earth  stretches the upwards perspective compared with a totally flat earth and would make an already sensitive observation of position with respect to the beam even more sensitive.  This is similar to the perspective illusion on a long straight road (say a mile or more) going into a long shallow dip (say around 100 feet).  The valley seems to appear much deeper and the rise at the other end much steeper.
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