Last Fall 2010/Winter 2011, I taught the science of weather and climate to non-scientists at Toronto’s York University.
During the Fall semester, a unit of NATS 1780 focused on atmospheric optics. Not surprisingly, rainbows were one of the topics that received attention.
By the end of this unit, students understood that rainbows are the consequence of a twofold optical manipulation of sunlight:
- Raindrops bend sunlight. Not only do raindrops bend (refract) sunlight, they do so with extreme prejudice. Blue light gets bent the most, red the least. In other words, this is a wavelength-based prejudice: The shorter the wavelength, the more the light is bent. This highly selective refraction is known as dispersion. Like a prism then, raindrops allow for the individual colours that comprise visible light to be made evident.
- Raindrops reflect sunlight. Inside the raindrop, reflection occurs. In fact, multiple reflections can occur. And if all of the angles are just right, these reflections can remain contained within the raindrop. This is known as the phenomenon of Total Internal Reflection (TIR).
If it were possible, how would a tertiary (i.e., third)rainbow be produced?