Unveiling the Physics of Double Rainbows
- Anaya Gupta
- Oct 23, 2024
- 3 min read
Updated: Oct 30, 2024
What is a double rainbow?
Secondary rainbows appear due to a phenomenon that is similar to the primary rainbow, with one big difference: the light that enters the raindrop and refracts at the surface does not escape after hitting the back of the rainbow. Instead, that light is refracted a second time, creating the secondary rainbow.
How does the double rainbow form?
Double rainbows are formed when sunlight is reflected twice within a raindrop with a violet light that reaches the observer's eye coming from the higher raindrops and the red light from lower raindrops.
There are a few key differences between primary and secondary rainbows.
First, the colors of the secondary rainbow are inverted from the primary rainbow. That’s right: the colors of secondary rainbows are the opposite of primary rainbows, moving from violet, indigo, blue, green, yellow, orange, and red (VIBGYOR). This is because light enters from the bottom of the drop instead of entering from the top.
You’ll also notice that this rainbow is not as bright; secondary rainbows are always fainter and harder to see than primary rainbows, this is because;
The amount of light that is refracted a second time (rather than escaping the raindrop) is much less than the amount of light responsible for creating a primary rainbow. Some of the light is refracted out of the raindrop at first the point reflection. Also, some light gets absorbed during reflection.
Violet and blue light have a tendency to get deflected at a greater angle than red light and vice versa. So, when the light rays get reflected twice in the droplets before getting refracted, there is a greater amount of distance between the outermost colours of light rays and the angular difference between the rays increases. This shows that the secondary rainbow is actually thicker than the primary one.
But, since a lesser amount of light is being spread in a larger area, this thickness is not visible because of the low intensity of light.
If we could see all 7 colours dispersed from each and every raindrop dispersing light in the sky, would it still look like a rainbow?
The individuality of the colours would not be visible as all the colours would merge within each other.
Seeing a rainbow from the human eye
A rainbow is an optical illusion—it does not actually exist in a specific spot in the sky. The appearance of a rainbow depends on where you're standing and where the sun (or other source of light) is shining.
When water drops are present within the air and sunlight shines from behind the observer at a low altitude angle, a rainbow is visible.
Light entering a rain droplet bends at specific angles, depending on its color. In a primary rainbow, red light exits at an angle of 42 degrees, while violet is slightly smaller at 40 degrees, whereas in a secondary rainbow, violet light exists at an angle of 52 degrees and red light at 50 degrees.
In a primary rainbow, the raindrops present higher in the sky, refract red light into the eye and the lower ones refract colours in the inner arc of the rainbow like violet. Vice-versa for a secondary rainbow. The different angles from multiple droplets form a complete circle of colour in the sky.
All raindrops refract and reflect the daylight within the same way, but only the sunshine from some raindrops reaches the observer’s eye. Each of us only sees the rainbow formed by those raindrops that happen to be at the correct angular direction to send the different colors into our eyes. So each of us receives our own special rainbow created by a dedicated set of raindrops in the sky.
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