Lenses that don't refract

From: Nicholas Bodley (nbodley@world.std.com)
Date: Wed Mar 28 2001 - 12:48:04 PST

This one really amazed the daylights out of me. When laser pointers
became inexpensive enough to meet my budget (someday, green ones will
be:), I aimed the beam from mine through an ordinary double-convex
magnifier lens, expecting the beam to diverge a lot. To my utter and
speechless astonishment, the beam passed right through, making a spot
on the wall apparently the same size as without the lens. Someone or
somebody clued me in a bit about Gaussian beams; I'm still in the
dark without a light. I'm mystified! My childhood-era explanations of
refraction (which I thought were correct) flew out the window.

I know that the laser diode's beam diverges a lot, and requires
refractive optics to collimate it into a beam. The collimator lenses
are quite small in diameter, I'm just about sure, maybe 2X the exit
beam diameter?

I suppose the spatial coherence of the beam, going crosswise, is
extremely good; that might be a starting point. Look into "coherence

Nicholas Bodley |@| Waltham, Mass.
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