Reconstructing Johann Wilhelm Ritter’s Discovery of Ultraviolet Radiation: A Photochemical Demonstration for Teaching the Interaction of Light and Matter

On February 22, 1801, Johann Wilhelm Ritter experimentally demonstrated the existence of an invisible radiation beyond the violet region of the visible spectrum. Using the photochemical darkening of horn silver (silver chloride) as an indicator, he provided the first evidence of ultraviolet radiation. On the occasion of the 225th anniversary of this discovery, Ritter’s original experiment was reconstructed and further developed as an accessible classroom demonstration. A major challenge in reproducing the historical experiment is the preparation of a homogeneous light-sensitive silver chloride layer. To address this issue, a simple two-step spray deposition method was developed that produces a uniform and reproducible silver chloride coating on filter paper. When exposed to a continuous spectrum generated by a halogen lamp and dispersed by a glass prism, the prepared paper exhibits a distinct darkening gradient in the short-wavelength region, with a pronounced intensity maximum at wavelengths shorter than the violet edge of the visible spectrum. The reconstructed demonstration enables the indirect detection of ultraviolet radiation through its photochemical effect and thereby facilitates exploration of the wavelength dependence of photochemical processes. In addition to illustrating the interaction of electromagnetic radiation with matter, the experiment provides an instructive historical example of how invisible natural phenomena can be inferred from chemical indicators. The demonstration is suitable for secondary school and introductory university courses and offers an interdisciplinary perspective on the historical development of spectroscopy and photochemistry.