The greater the number of slits, the better the definition of minima and maxima.Ī front view of a spring is made up of two series of segments having a diameter equal to the diameter of the spring wire, spaced apart on a regular basis, with pitch P, and angled together with angle 2α. The figure below shows the diffraction pattern obtained from n slits : the minima with greater spacing corresponds to the width w, while the minima with smaller spacing correspond to the distance d between the slits. The theory of slit diffraction is well known. This principle helps us because the diffraction pattern produced by a set of slits of width w and spaced d is the same produced by a set of wires having a diameter w and spaced d. In practice, the diffraction patterns produced by an obstacle and by an aperture having the same linear dimensions are the same. However, let us recall the principle of Babinet, which states that the diffraction pattern produced by an opaque disk of diameter D is identical to that produced by a circular aperture of diameter D. We will not deal with diffraction in detail, for which we refer to the numerous texts and the following previous Posts : Laser & Diffraction Grating, Light as a Wave : Slit Diffraction. Diffraction is the basis of many technologies, scientific techniques and common optical and electromagnetic phenomena. The diffraction and interference of electromagnetic radiation are easily observable phenomena that give direct and tangible evidence of the wave nature of light. Our “simulation” with laser and spring, will allow us to understand how from the diffraction images (an example is shown in the cover image) it is possible to obtain detailed information on the spatial structure of the DNA molecule. Of course working with X-rays and with “crystallized” DNA molecules is not trivial, for this reason our experience will be a sort of simulation made with the visible radiation of a He-Ne laser instead of X-rays and with a spring (a macroscopic spring) instead of the DNA molecule. This experimental research, conducted at the time by Rosalind Franklin, allowed us to understand the structure of the DNA molecule. In this Post we describe our attempt to replicate the experiment on X-ray diffraction by the DNA molecule.