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