When a green laser pointer beam (532-nm wavelength) is directed into a clear tube filled with a popular cola drink, the light path appears yellow, orange, or even deep red, if the room is dark enough, in sharp contrast with the green color in air, water, and the container walls (see Fig. 1). We seek to explore the physical reasoning that, starting from laser monochromaticity and a darkened environment, leads to frequency-changing fluorescence as the most plausible explanation.1 We have observed the effect in cola drinks having in common just the caramel-based food coloring E150d, while the effect is absent in some other drinks without this ingredient. This led us to try the effect in home-prepared caramel dissolved in water, obtaining a more pronounced effect for large concentrations (see Fig. 1).
The frequency-changing fluorescence hypothesis can be tested in several different ways, such as spectrum observation with...