In November 2003 I published a short news story in Scientific American about the orbital angular momentum of light and how you can glimpse it with a laser pointer and an overhead transparency. The article is no longer available at scientificamerican.com, so, as a service to fellow makers and citizen scientists, here are the instructions:
- Download the diffraction grating pattern from Enrique Galvez’s website at Colgate University. The fork at the center of the pattern is what twists the light. (If that link is broken, try here or here.)
- Using a photocopier, reduce the pattern to about half a centimeter on a side and transfer it to an overhead transparency. Make sure the fork doesn’t get smudged. (Finding a transparency in the age of PowerPoint is probably the hardest part of the experiment.)
- Shine the laser through the pattern, ensuring that the beam passes through the fork, and project it onto a wall a few meters away. The grating splits the laser beam into a row of circles.
Each of the circles flanking the central circle should have a small hole in the middle. The holes are a sign that light is being twisted. If you move the laser beam off the fork, the holes go away.
Could you focus the dot by adding a coil with current running thru it?