By Patricia Daukantas
Three physicists have figured out how to recreate a famous X-ray-diffraction experiment with a laser and other simple equipment. Their goal is to enable undergraduate students to follow in the footsteps of a chemical physicist who helped to decode the structure of DNA.
Rosalind Franklin (1920-1958), a young British scientist, took the famous X-ray diffraction image that was critical to identifying the structure of DNA as a double helix. Heidrun Schmitzer, Dennis Tierney and Gregory Braun of Xavier University (Cincinnati, Ohio, U.S.A.) include Franklin in their undergraduate course for non-majors on “Women Who Shaped Physics.” Featured scientists in the course include Marie Curie, Lise Meitner, Jocelyn Bell Burnell and Maria Goeppert-Mayer.
In their poster paper at this week’s American Physical Society March meeting in Portland, Ore. (U.S.A.), Schmitzer and her colleagues described the classroom experiment, which requires only simple tools: a red laser and the spring from a retractable ballpoint pen. Shining the laser beam through the spring projects a diffraction pattern strikingly similar to Franklin’s famous image. See the Xavier group’s photo of diffracted light and compare it to the X-ray image from 57 years ago (and an accompanying mathematical analysis).
By comparing the geometry of the pen spring to the diffraction pattern of the light, and then studying the Franklin X-ray image at its original size, the students “can determine the angle, pitch and radius of the DNA molecule, just like Rosalind Franklin,” Schmitzer wrote in the abstract.
Last night I did a quick trial of this with a spring from an old pen, my cats’ favorite laser pointer and a darkened room. Unlike Schmitzer, I did not block the bright center maximum with anything, so my result wasn’t as visually stunning. But I could see some evidence of the “X” pattern with faint characteristic stripes. I suspect that, with a bit more equipment and refined technique, this could make a stunning classroom demonstration.
2010-03 March, Biomedical optics, Optics history