It was an awkward moment recently when, working on an activity with my kids at our coffee table, I suddenly saw something on my nose that I could not get off! The apparent phantasm was colorful, but seemed to disappear and reappear with my every move. A tome of Lovecraftian tales rushed through my mind. Had I become a target of some eldritch iridescent stain that will mar my family for generations? Lo and behold, the children pointed out that a prism on our window was refracting sunlight. The light then dispersed into the different wavelengths of visible light, making a rainbow that became projected onto my face. Hm. Maybe the Halloween spirit is getting to me...
But how does sunlight become a rainbow? Refraction is the bending of a wave, whether it is light, sound, or water. The wave bends due to a reduction in speed which, in the case of light, occurs due to a change in the density of the medium through which the light travels. In my situation:
1) Sunlight was cruising through the atmosphere
2) The light hit my living room window and a prism hanging on it
3) Light waves slowed down and bent as they entered the prism
4) The changed wavelengths of visible light separated into different colors
5) A rainbow was projected on my face
When you see a rainbow outside, the sunlight is being refracted and dispersed by water droplets in clouds.
The study of light refraction goes back nearly two thousand years, to a time when Ptolemy, a Roman scholar in Alexandria, Egypt, explained that light originates within our eyes. The first known mathematical explanation of refraction was published by the Persian mathematician, Ibn Sahl, around the year 984. Soon after that, an Arab scientist, Ibn al-Haytham, revolutionized his world when he stated that vision is the result of light entering the eye. It's easy now to think that a lack of bright beams shining out from the eyes of literally anyone should be enough evidence that light does not come out of our eyes, but hey, those old-school scientists worked hard with the tools they had to transform philosophical ideas into the scientific principles that we know and love today. However, it was not until centuries later, in the 1600s, that the idea of refraction struck a number of European thinkers. Thomas Harriot wrote to Johannes Kepler about it, but never published his ideas. Willebrord Snellius (Snell), for whom the law of refraction is named, mathematically derived the very same principles as Ibn Sahl, but, like Harriot, did not publish his thoughts during his lifetime.
Not long after Snell's dwellings, René Descartes pondered the possible causes of light providing an apparent rainbow after passing through a glass of water. Isaac Newton’s experiments with prisms in 1666 provided evidence that the colors are present in the light already and not derived from the medium through which the light passes. In 1678, Christiaan Huygens published his principles of wave propagation, which helped to mathematically explain the behavior of light as a wave. Huygens discovered that as a wave passed through a medium of higher density, the front of each wave became compressed. The wave must bend in order to stay connected across the medium's boundary. All of this intensive scrutiny by European scientists occurred within the 1600s. What a heavy hundred years of light study! Huygens’ explanations seemed to be about the extent of human ability to understand the bendy behavior of light until 1801, when the double-slit experiment confirmed the wave nature of light.
Science is a process, y’all! That’s right, when deployed properly, science is a verb. Do science in your daily life by looking out for interesting natural occurrences and ask “What makes that work?” Chances are that in your exploration you’ll be able to find a long list of heavy thinkers who wondered the very same thing. Congratulations, you’ve scienced with the best of them!
Prism: By Zátonyi Sándor, (ifj.) Fizped - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12226449
Golden Gate Bridge: By Brocken Inaglory, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3962888
Ibn Sahl excerpt: By Ibn Sahl (Abu Sa`d al-`Ala' ibn Sahl) (c. 940-1000) - Milli MS 867, fol. 7r, Milli Library, Tehran, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3303465
Refraction animation: By Ulflund - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=73784342
Rainbow nose: Selfie, cutout with GIMP