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.
- 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.