Laser head patterns

I was back in Austin for Thanksgiving last week. While hanging out in the room where my dad tutors his students, undoubtedly chatting with him about what on earth I’ll do next year, I started playing with his laser pointers. He has two, one of which has multiple heads that you can screw on to get different pointer shapes (like this one). There are heads for an arrow, a circle, a line, and a cross, in addition to the normal pointer. The heads look like basically like a piece of glass in a metal frame, and I wondered: What are the patterns on the glass that produce these different pointer shapes?

arrow circle line

Miraculously, I also happened to notice that my dad now has a little microscope! So I plunked the laser pointer heads underneath it, to try to see what they look like.

And I can’t say that I understood what I saw. For the line-shaped pointer, I saw lots of lines etched in the glass, seemingly in parallel:

head head

But with the arrow-shaped pointer head, I saw something very weird:

arrow

…No little arrows all over, but rather, a pixelated etching of wavy lines! All I can imagine is that this is something like one of those Magic Eye pictures where an image appears when everything lines up just right.

Does anyone know how this works?

November 29, 2007. how things work. 5 Comments.

5 Comments

  1. Rachel replied:

    I think they’re diffraction gratings. The line one sounds like the classic diffraction grating pattern: a bunch of parallel slits.

    This link on Fresnel diffraction sounds like it may be related to the arrow one. Maybe the little squiggly lines create things that superimpose into the arrow.

    I can’t find anything on the internet to back this up, other than this sketchy patent site.

    November 29th, 2007 at 2:54 am. Permalink.

  2. brent replied:

    Ha, and here I just figured it was a little arrow cutout. Wow, go science.

    November 29th, 2007 at 11:36 am. Permalink.

  3. Anita replied:

    Ah! Thanks to Rachel’s hints, and a suggestion by Judy Brown, whom I met today, I found a page that describes the phenomenon in several parts:

    So this is like a hologram? I’ve gotta find a better explanation, but that will have to wait until the weekend.

    November 29th, 2007 at 8:23 pm. Permalink.

  4. Rachel replied:

    Damn, now I wish I’d done the hologram lab in physics lab! Who knew I was actually going to use it some day (for a very loose definition of “use”)?

    This page on computer-generated holography is kind of hilarious. Silly grad students.

    November 29th, 2007 at 11:12 pm. Permalink.

  5. Brandon Roy replied:

    Kind of related… I learned that lenses do a Fourier transform. I thought that was pretty cool – so if you have a spatial pattern of alternating black and white lines at a certain spatial frequency, there is a point behind the lens where you get a dot, related to that frequency. And if you have interleaved lines of different frequencies, you get multiple dots! All of this is hearsay, but I think its true…

    November 30th, 2007 at 5:10 pm. Permalink.

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