Visualizing Light Rays Using Fluorescence

We present a simple and effective experimental setup to help students visualize light rays in optics experiments. When illuminated by a blue-violet laser pointer (typical wavelength [Formula: see text][Formula: see text]nm), tonic water and phosphorescent paper show fluorescence which allows the laser beam to be clearly seen by a large audience under normal ambient lighting. The setup is particularly useful for conducting classroom demonstrations on the reflection, refraction and diffraction of light. Quantitative measurement is also possible, e.g. students can attach a transparent scale to the phosphorescent paper or container holding the tonic water, and perform a quick verification of Snell’s law and the diffraction formula [Formula: see text]. The appeal of the visualization of light beams in multiple total internal reflections and retroreflection could help teachers introduce technological applications such as optical fibers and retroreflectors in road signs. A simple but interesting experiment of scattering laser light to form a light cone in a medium is shown using the fluorescence method. Redoing or modifying other innovative optics experiments using the fluorescence method is also discussed.