Re: Laser beam

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

The transparent plate "mystery" (see below) is not a mystery
anymore. It has nothing to do with the laser. By placing the
plate in front of my eye, and looking at a lamp 25 feet away
I see the main image and numerous "images" due to inner
reflections. This explains the line. Trivial geometrical optics?
Yes, but it was important to clarify this. Similar lines in
calcite will no longer bother me, I will ignore them as
unavoidable background.

> Our ordinary 0.5 mW He-Ne laser beam has the diameter of
> about 1 mm; it is reasonably parallel. A clear round spot is seen
> on a piece of white paper when the beam is intercepted on it,
> about 1 meter away from the laser. No lenses, no mirrors,
> no pinholes are used.
>
> A transparent glass plate 10 mm thick and 5 cm wide is inserted
> into the beam (several cm from the laser) in such way that the
> beam travels 5 cm. It enters the plate through one polished edge
> and exits through the other polished edge at 90 angle. The two
> edges are parallel. The picture I see on the screen (1 m away)
> still shows the bright spot but it also shows a straight line which
> is at least  10 cm long. The line is vertical when the face of the
> plate is horizontal, it becomes parallel when when the face is
> vertical. (the line is parallel to the normal of the face)
>
> I took another plate (only 5 mm thick) and observed an identical
> straight line on my paper screen. In other words, the orientation
> of the plate can be determined from the orientation of the line
> crossing the beam spot. Can somebody confirm this? What causes
> this phenomenon? How to get rid of the annoying straight line? It
> prevents me from doing something more interesting. Ordinary
> scattering would produce an axially symmetric background, not
> a straight line. By the way, a slightly wider beam from a
> semiconducting laser pointer behaves the same way.
> Ludwik Kowalski