Rainbow as a natural phenomenon is the result of the dispersion of light on water droplets in clouds. It was established in the 14th century that a single drop of water is enough to analyze the formation and the properties of the rainbow. The largest contribution to the properties of a rainbow formed in nature comes from small raindrops whose shape is approximately spherical. Therefore, in this paper, a spherical flask filled with some liquid was used as a simplified model of a raindrop and an overhead projector was used as a source of parallel light. The theory of rainbow formation was derived from elementary laws of geometrical optics with the inclusion of the vessel wall thickness effects. These effects due to the additional scattering on the walls of the vessel affect the changes in the position of the rainbow, the angular width of the rainbow, the angular width of Alexander’s dark region and its splitting. Numerical simulations were used to make theoretical predictions with which the experimentally obtained values were compared. The liquids on which the measurements were made are water and ethanol, and the measurements themselves were performed with a DCC1240C-HQ camera (THORLAB) and a Xiaomi Pocophone f1 M1805E10A mobile device. Image analysis has shown that the results follow predicted dependencies but are inaccurate. Despite large inaccuracies, it was concluded that the experiment is still useful for the purpose of qualitative observation and the demonstration of rainbow formation for educational purposes.