Rainbows can be a bright spot after a storm rolls through. They are not something you can reach out and touch or see for a long period of time. Rainbows are an optical phenomenon that has to deal with physics and how optics can impact viewing if the necessary atmospheric conditions are present. Those necessary ingredients include having the sunlight to your back and a shower or storm in front of you.
Our eyes see sunlight in our atmosphere as a white light. That white light actually contains all the different colors of the visible light spectrum: red, orange, yellow, green, blue and violet. It takes other objects such as water droplets suspended in Earth’s atmosphere to bend, or refract, the sunlight as it passes through a water droplet to scatter the sunlight into the various vibrant colors that make up the rainbow.
Water has a higher density than the air surrounding it. When sunlight, traveling at a constant speed through the air, comes into contact with that water droplet, the speed at which the light is traveling will slow down. The sunlight then bounces off the water droplet then the beam of light bends back before the light exits the droplet. This bending causes the light to scatter into its various separate wavelengths, or colors.
The density and optical changes cause spectators to see rainbows. There are certain times of day that rainbows are visible to our eyes at ground level: sunrise and sunset. We cannot see rainbows at all times of the day, even if the sun is shining with showers and thunderstorms in the region. The lower sun angle is critical in having the necessary optics for all of this to work.
It has been calculated that a 42° angle between the sunlight beam and the viewer is the magic ticket to seeing the rainbow. As the sun rises or sets, the perfect arc will start to disappear and fade in segments. If one were to view a rainbow from a higher altitude in the sky with that 42° angle maintained between the viewer’s sightline and the sunbeam, one would find that the rainbow creates a perfect circle. However, the ground cuts off the bottom half of the rainbow meaning we can only see the familiar arc in most cases.
How large or small the water droplets are that the sun is reflecting against will impact how vibrant or dull a rainbow appears to the human eye. Larger water droplets are associated with a more tropical atmosphere and will make a rainbow appear thinner and brighter while smaller water droplets in the atmosphere will contribute to a wider, more dull-looking rainbow.
Double rainbows are the result of a secondary reflection of the primary rainbow. Due to this double reflection, the colors are not as vibrant in this second bow and it is located 9° above the primary rainbow. The reflected rainbow is less vibrant and the width is typically larger than the primary rainbow.
The colors are also in reverse order due to the reflection. Due to additional light scattering, one will also notice that the sky inside the primary rainbow tends to look brighter. Rainbows are truly a beautiful optical phenomenon that are amazing to capture every time one occurs. As always, you can submit your weather photos to us here, we love to see the sights captured from across the Sunflower State!
— Meteorologist Erika Paige