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I'm new to optics, especially Gaussian beam optics. I'm curious about things that I've been reading in Daniel Steck's textbook, and how they compare with my common sense intuition. Guassian beams are commonly used to model laser light, and typically are depicted according to this picture

Gaussian Beam

We can clearly see dispersion (or the spreading of the light) as we move along the z axis away from the waist (w0). But, as I think about the lasers I use for pointing at stars for instance, they seem intuitively to have some diameter which remains constant as the light shines away from the source. In other words, intuitively it seems that there is no dispersion or spread for a laser I use to point at stars. Or is it just that the dispersion is very small so that I can't notice it? Any help would be appreciated. Thanks

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    $\begingroup$ Why are you pointing lasers at stars??? $\endgroup$ – aquirdturtle Jul 31 '16 at 21:09
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    $\begingroup$ Calculate $b$ for a typical laser. Take $w_0$ to be 3 mm or so. $\endgroup$ – garyp Aug 1 '16 at 1:38
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The dispersion is definitely there, but too small to notice at that scale. If memory serves me right, a typical laserpointer has a dispersion of about 0.2 mrad, resulting in a dot size of about 20cm at 1km range.

You can counter the dispersion by using a wider beam to start with, but you can never completely negate its effects.

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