Re: Mobile Speed Camera Vans: anything changed recently?

"M.I.5?" <no.one@xxxxxxxxxxxxxxxxxxxxxx> wrote:
<snip>
The general principle is used to measure the distance to the moon using a
reflector left by the Apollo missions. The laser beam once it has travelled
the half a million miles or so round trip is still only 5 feet or so in
diameter.

Err, no.
The beam spread is 1.22* wavelength / diameter.
Where wavelength is the wavelength of the light, and diameter is the
diameter of the final optic.
A million Km with a beamspread of 2m, is a ratio of 1/500 million, which
means you need an optic about 600 million wavelengths across.
For red light, say 620nm, this is about a kilometer across.

Not quite your ordinary laser pointer.
Actually, this can be observed quite easily.
Rubber-band down the button of a laser pointer, and shine it at a
distant object.
Walk over to the distant object, and you'll see that the beam is muich
larger.
Diffracton on most laser pointers is about 1/1000 radian, or at 100m,
10cm.


.

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