Rainbow Science

With Saint Patrick’s Day around the corner, let’s demystify the rainbow! No, it is not magical, the beauty of this color spectrum exists because of science!

Quick History of Rainbow Science

Rainbows have fascinated many throughout time. A multitude of cultures have developed mythologies about rainbows that are still known, and read about today. Scientific explanations started with the Greeks. The Greek philosopher, Aristotle, was the first person to qualitatively describe the marvel of the rainbow. Aristotle described in an organized and detailed manner what he saw: the colors present, the order of the colors, the times of day a rainbow was present, and the maximum number of rainbows that were present at one given time. A more quantitative and scientific description was given by Qutb al-Din al-Shirazi, a poet and mathematician from Persia in the Arabic Middle Ages. His description was mathematically backed, based on the science of refraction, discovered by another Persian mathematician.¹ Their description of the rainbow was defined by “…the water drops composing the clouds reflect the light ray and create the rainbow’s colours through a refraction and two or more reflections…”.¹ Marco Antonio de Dominis was the first to completely explain the phenomenon of rainbows, but Isaac Newton was able to prove the concept, which was shown in his book the Opticks: Or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light. Newton showed that when light passes through a glass prism, the light is separated into the color spectrum seen in a rainbow. Newton’s research was further developed by Thomas Young, who described the wave-like behavior of light. The more modern descriptions of the phenomenon are due to various scientists who worked endlessly in the 20th century.¹

Breaking Down the Main Components of Rainbows

Human eyes are able to see a certain kind of light, called white light (also known as visible light). White light is made from the combination of colors: red, orange, yellow, green, blue, indigo and violet. There are many other types of light that exist that humans cannot see. For example: ultraviolet, infrared, and X-ray light.²

Light is the fastest moving thing known to man. The speed of light (c) is equal to 3×10⁸ meters per second (m/s). To put that in perspective, the fastest car in the world hits a top speed of around 300 miles per hour, or 134.11 m/s. Meaning that light travels over 2 million times faster than this car! This speed refers to how fast light can move through air or space in a vacuum, not how light moves through other materials, like glass or water. When light moves through glass, for example, it can slow down significantly.

Light waves are based on the laws of reflection and refraction. The angle a light ray will hit a smooth surface (like a mirror) will be equal to the angle the light ray reflects or bends off of the surface. Light scattering happens, when light hits a surface that is uneven, so the light ray angle that hits the surface is not equal to the light ray angle that reflects off of the surface. In refraction, light rays will not reflect off a surface, instead it passes through different materials. For example, the light ray will pass through one medium into another, like from air to water. Since light is traveling through different materials, the speed at which it is traveling will change as well. So, the angle that the light bends or refracts depends on the difference in speed that the light ray experiences when changing mediums.³

So What Causes a Rainbow?

This optical phenomenon can only occur in the right weather. It will usually happen right after rain, when water droplets are present in the air, and the sun is peaking behind the clouds. Sunlight, also known as white light, is made up of many wavelengths or colors. When the sunlight hits a water droplet, the light ray is traveling from an air medium to a water medium, changing the speed as it travels, causing the light to refract. When the light refracts, certain wavelengths refract more than others as the light travels into the water droplet. Red has the largest wavelength, while violet has the smallest wavelength. Red refracts the least, while violet refracts the most. This is why when we observe a rainbow red is at the top of the arc and violet is at the bottom.⁴

Every time you see a rainbow now, whether it’s in your cereal, or in the sky, you will know the science behind why we see the beautiful spectrum of colors!

References

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