Demonstrating Mechanical Beats on Phosphorescent Foil in Real Time

The interference pattern between two harmonic oscillations with slightly different frequencies are called beats. The beats, as a combined motion, have two different periods, one approximately equal to the period of the original oscillations, and another that is significantly longer and is related to the variable amplitude of the motion. The main objective of this presented work is to engage students' attention and demonstrate in a simple way the concept of mechanical beats as a resultant motion of combined oscillations, but also to experience it through a visual projection. The proposed experimental setup is constructed in a way that satisfies both objectives and it consists of two parts that allow us to demonstrate in real time the combination of two harmonic motions. In the first part, the resultant motion is generated by the oscillations of two parallel rulers equipped with mirrors, and in the second part the visual demonstration of the beat occurs on a phosphorescent rotating foil by using a blue violet laser beam.