How Rosalind Franklin Discovered the Helical Structure of DNA: Experiments in Diffraction

The Physics Teacher ◆ Vol. 49, March 2011 DOI: 10.1119/1.3555496 Experiment 1: Why does the x-shaped pattern reveal the helical structure? This is a variation on the single-slit experiment with readily available equipment, allowing the student to both see a diffraction pattern similar to Franklin’s and to determine the pitch angle α of the helix, as in Fig. 2. If you take the spring from a typical retractable pen and place it on an overhead projector, you observe that the projection of a helix is a sine wave pattern (Fig. 2). We place a laser pointer6 or a diode laser (from PASCO) on one end of the lab table and direct the beam (diameter ~2 mm) through the side of the spring. (The distance between the laser and spring does not matter.) Holding a white card a couple of centimeters past the spring shows the projection is indeed a sinusoidal pattern and also reveals the number of illuminated pitches. (The shadow will not look sinusoidal if the helix is not oriented perpendicular to the beam.) Since the diameter of the beam is small, it will only illuminate one pitch, i.e., one turn of the helix and only the relatively straight parts of the sine wave, not the curved maxima or minima. We project our diffraction pattern on a screen or wall at least 2 m away, with a larger distance (~4 m) better, since it gives a larger pattern. A mirror can be used to fold the path, allowing for a larger distance on a shorter table. Babinet’s principle states that the diffraction pattern of an obstacle is the same as the diffraction pattern of an aperture of the same shape.7 According to this principle, the diffraction pattern formed by the two straight sections of the wire (one on each side) is equivalent to the diffraction pattern of two single slits oriented at a certain angle with respect to each other (see Fig. 2). When comparing the diffraction pattern of the helical spring (similar to Fig. 4) with the x-ray diffraction pattern of How Rosalind Franklin Discovered the Helical Structure of DNA: Experiments in Diffraction