How is a double rainbow created?
In our latitudes, we see a rainbow particularly often in the month of April, when short, intense showers alternate with the sun. The direction in which it can be seen can be determined precisely because it is always at an angular distance of 42 degrees from the point that is exactly opposite to the sun. For example, at midday you never have the opportunity to see a rainbow because the sun is so high that the point opposite to the sun is below the horizon, so a rainbow is not visible to us.
From the angle you can generalize that the higher the sun is, the deeper and flatter the rainbow is and the higher and more curved the lower the sun is - for example in the evening light or morning light. The reflection angle is always 42 degrees. This is because the light rays that enter a drop of water are always reflected by the layer between the water and the air layer at a certain angle to their direction of incidence, although from a physical point of view no deflection of more than 42 degrees is possible. The rainbow that is then visible to us is created by the light rays that would have to be refracted further than these 42 degrees and concentrate at this limit.
The rays of sunlight have different wavelengths, and since the refraction of light differs depending on the wavelength, the light is not only deflected, but is also broken down into different color components. Since, for example, red light is refracted weaker and blue light is refracted more strongly, the latter is in the direct vicinity of the original beam when the droplet emerges. This means that the blue stripe of the rainbow is always on the inside and the red stripe is always on the outside of the rainbow.
Sometimes you are lucky enough to see a double rainbow.
The rainbow that is stronger in color and lies on the inside is called the main rainbow, while the rainbow that is slightly weaker in color and larger and on the outside is called the secondary bow. This appears like a mirror image because its color sequence is exactly the opposite of the main arch. The color of the secondary arch is much weaker because the light rays in the secondary arch are reflected not just once, but twice, so that their quantity is smaller than in a simple main arch.
In a single rainbow, sunlight is refracted by raindrops and is simply reflected, while in a double rainbow there is a double reflection. In the secondary arc, sunlight is also refracted into rain droplets, with parts of the light beam being reflected twice in the drop at the interface between water and air, depending on the angle of entry. Here the limit value is 51 degrees for the angle.
When leaving the drop, the light rays cross the original direction of sunlight. The blue ray is again refracted the most and forms the larger angle after crossing, which means that the blue ray is on the outside of the secondary arc and the red is on the inside. Its color sequence therefore appears like a mirror image of the main arch.