Science Behind the Rainbow

Rainbow cover

Rainbows are one of the most beautiful natural phenomenons that fascinate us from our childhood to old age. It is something that never gets boring, each time we come across a rainbow we are just automatically attracted to it and in the age of smartphones, you bet there will be a lot of photographs taken of this event. But why do rainbows even happen? Let’s look into the science behind this appealing phenomenon.

Physics behind the Rainbow

I’m certain we’ve all studied in fifth grade how the rainbows are created, however, unless one picked physics as their subject in high school the detailed explanation is often missed. So the basic explanation is that when sunlight hits a water droplet it is refracted within the water droplet and then it is reflected back and leaves the water like a rainbow, this is true but there is some more aspects involved in it.

Eye Rainbow

An eye reflecting seven colors symbolizing visible light.

  • Visible Light: Light is the essential aspect of visibility without it we wouldn’t be able to see anything at all. Light in itself is photons traveling that have the properties of both a wave and a particle, these travel at a speed of 299792458 m/s, which makes light the fastest thing in the universe. Although the entire spectrum of the light consists of other forms of light such as gamma rays and x-rays, our eyes are only able to perceive a tiny fraction of it known as visible light. Also, the sunlight in this scenario is a combination of seven colors known as VIBGYOR, violet, indigo, blue, green, yellow, orange, and red light depending on their wavelength, it is just that when sunlight is refracted it is split into fundamental wavelengths resulting in the rainbow.
  • Critical Angle: As we know that the nature of light is to refract away from the normal (an imaginary line perpendicular to the surface where the light hits the surface) when it is shined from a denser medium to a rare medium. When the angle of the light being refracted in the rarer medium becomes 90° degree, this angle is known as the critical angle. It should be noted that whenever a rainbow is created, light is always below the critical angle in both refraction and reflection.
  • Total Internal Reflection: Some people are under the notion that all of the light that is refracted from the water droplet is reflected back within the droplet. However, that is not the case, if that were to happen we’d only see rainbows and nothing else.

So, going back to the way rainbows are created, sunlight is hit upon the spherical surface of the water at less than the critical angle and from there sunlight is refracted and some of it reflected away. As the light is moving forward some of it is reflected back from the interior of the water droplet again at an angle below the critical angle and some of it is again refracted out, while the light that was reflected back refracts again and split into seven colors of the rainbow that we see due to different wavelengths. Also, the intensity of the rainbow depends on the angle where the light hits the water, 42° produces the most vibrant rainbows.

The curvature of the Rainbow: Rainbows are always arched, wonder why is that so? It has nothing to do with gravity as some suggest, although gravity is responsible for making the water drops spherical. Since there are countless drops of water that are producing the rainbow, it is the different angle of the individual drop from our eye which is stacked in a way that appears one higher than the other and then falls after a peak due to our angle of seeing them, that’s why rainbow appears curved.

Curvature of rainbow

Arched rainbow over the ocean.

Other forms of Rainbows

  • Pilot’s Glories: Unlike the typical rainbow, these are observed by pilots and passengers as well when looking at the shadow of the airplane on a mountain or on the clouds. Physicists say that these are formed because the mist particles are extremely tiny compared to raindrops, raindrops are usually 1 millimeter in size while the mist particles are 1 micrometer. The exact physics of these rainbows is one of the most complex ones in the realm of optics and as an individual who doesn’t understand such complex physics, it is hard to explain or even understand.
Pilots glory

A photograph showing the pilot’s glory.

  • Fogbows: These awe-inspiring bows are the result of tiny fog particles interacting with the fog particles which are nearly the same size as light’s wavelength. When this happens a phenomenon is known as diffraction, where when the light hit the surface of these particles, it doesn’t reflect or refract per see but more like deviating from its path and whatever colors that would result in a rainbow, overlaps each other resulting in the white halo-like bow.
Fogbow white

A fogbow appearing over the Arctic.

As you can see rainbow even after we’ve studied about it in school and seen it countless times is still one of the most fascinated topics to both look at and learn about. The science behind nature is truly mesmerizing.

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