Rainbows are produced by the combined effects of refraction, reflection, and dispersion of light in water droplets. Much of what we know about the physics of rainbow formation was provided by René Descartes.1 He concluded that the light enters a spherical drop of water, is reflected inside the drop, and emerges at an angle of 42°. Thus, the rainbow appears to be a circle with an angular radius of 42°. Newton later provided a more detailed theory of the rainbow in his Opticks.2,3
An examination of rainbow production and the interactions that occur when light strikes a drop of water may be conducted by using a glass sphere, the type often used by photographers to produce special effects.4 In their 2016 TPT article, Ivanov and Nikolov describe a method of demonstrating rainbow formation using such a sphere and an LED flashlight.5,6 To increase the intensity...