Re: Can I punch holes so that a handheld laser pointer will diffract a pattern?

A good way to think about diffraction is that it starts to become a 
significant factor in the propagation of light when the diameter of the 
hole through which the light passes approaches the wavelength of the laser 
light.  For most laser pointers, the wavelength is about 650 nm or 0.65 
microns, so that you'll need to make a pinhole that's pretty small in 
order to see diffraction effects.  Try getting a really tiny, really sharp 
needle and making holes in aluminum foil.  Put the aluminum foil down on a 
fairly hard surface and poke the needle through it, then rotate the needle 
slightly to try to make the hole circular.  Direct the beam at the hole -- 
being very careful NOT TO LET THE LASER REFLECT INTO YOUR EYES -- and look 
at the pattern of the light coming out the other side and projecting onto 
a wall.  If diffraction is playing a role in the beam propagation, you'll 
see a series of concentric rings.  Another way to do this is to direct the 
beam at the edge of a metal razor blade, so that it's about half on and 
half off the blade.  If you look at the shadow of the beam on the wall, 
you should see some dark and light bands parallel to the edge of the 
blade - those are the diffraction maxima and minima resulting from the 
interaction of the beam with itself.

If these experiments are successful and you see the rings and fringes, 
then you might want to try to make an array of small holes -- try to make 
them about the same size -- within an area less than the beam diameter.  
Try to make them in various patterns:  in a square grid, in a grid with 
the points of each row above a blank area of the one beneath it, in a 
circle, etc.  If you're able to see interesting patterns, you'll be 
observing interference effects from the light coming from each of the 
holes.  A really interesting thing to do is to make 2 small holes.  Look 
at the interference pattern, then cover one of the holes so no light will 
come through.  When one hole is open, you will see the diffraction pattern 
of the hole; when both are open, you get an interference pattern resulting 
from the interaction of the light coming through both holes.  Note that 
some of the places that were light when only one hole is open become dark, 
and vice versa.

If you can't see any of these fringe phenomena at all, it's because your 
laser pointer has poor "spatial coherence" which basically means that it's 
not much of a laser, and that you should try the experiment at school 
where you can find a helium-neon laser to work with.  Those almost always 
are clean enough spatially to show good diffraction and interference 
effects.

Last, but not least, here's a neat optics demonstration that's the only 
macroscopic example of quantum mechanics that I know of -- and you don't 
need a laser... only 3 pieces of polarizing film or 3 sets of polarizing 
sunglasses.  First, take 2 pieces of polarizing film and "cross" them, so 
that the polarization coming through the first film is perpendicular to 
the transmission axis of the second and essentially nothing comes 
through.  This is easy to understand.  Now, if you take the third 
polarizer and put it so that it's rotated around the beam axis at some 
angle -- say 45 degrees -- to the other two, you see an amazing thing: 
even though the light had previously been blocked between the "crossed" 
polarizers, some light now passes through!  The reason this works is that 
polarization is an example of a quantized phenomenon.  The light going 
through the first polarizer is in a state perpendicular to that passed by 
the second, so nothing goes through.  When the third polarizer is 
introduced, it decomposes the state passed by the first polarizer into 
components parallel and perpendicular to its axes.  When this light enters 
the following polarizer, it has some component parallel to the 
transmission axis, so that light is transmitted out of the 3 polarizer 
stack.  Amazing!

Regarding filters, I really can't suggest much.  In general, filters 
absorb or reflect certain portions of the spectrum and, since lasers are 
generally pretty much a single color output, not too much happens except 
the light is either transmitted or not.

Let me know how your experiments turn out, please.


Steve