A simple experimental setup for measuring microparticle diameters with a smartphone and an LED

We present a simple and low-cost experimental setup for quantitatively measuring the size of microparticles in powders. The method requires only inexpensive, readily available materials and can be easily reproduced at home, making it particularly suitable for distance learning applications. The theoretical framework relies on Babinet’s principle, which states that, to very good approximation, a particle generates a diffraction pattern equivalent to that of a circular aperture. In practice, the powder sample is placed on a glass slide and positioned directly over the camera of a smartphone. A small piece of a diffraction grating is attached over the slide, and the resulting diffraction images are captured with the smartphone camera. The images are then quantitatively analysed using the open-source software Tracker. As light sources, light emitting diodes of different colours are employed. This accessible setup provides a robust and reproducible approach to introducing students to diffraction phenomena and particle size measurement.