Photo 51: X-Ray Diffraction & the Discovery of DNA's Double Helix (2D Model)

Rosalind Franklin's 1952 fibre-diffraction photograph of B-DNA is one of the most consequential single images in the history of science. This 2D simulator projects a real double helix — built from adjustable pitch, radius and inter-strand stagger — onto the canvas with a genuine depth-sorted orthographic projection you rotate by dragging, and computes, from the actual Cochran–Crick–Vand helical diffraction formula, the X-ray pattern that structure would cast on a photographic plate. Push the strand stagger away from its historical value of 3/8 of the pitch and watch the faint 4th layer line reappear in both the film and the intensity bar chart: that single missing spot in the real Photo 51 was Franklin's own measurement, and it was decisive evidence for a two-strand helix with ten base pairs per turn.