What shape is the rainbow seen from the plane on a sunny day?

The rainbow seen from the plane on a sunny day is flat.

Water drops are round, and the rainbow formed by sunlight reflection should also be round. Only when the observation on the ground is restricted can we see the circular rainbow.

Rainbow is formed by the scattering and reflection of sunlight on small round water droplets in the air. When sunlight enters the water drop, it will be incident at different angles at the same time and reflected at different angles in the water drop. The reflection of 40 to 42 degrees is the strongest, which produces the rainbow we see. When this reflection occurs, sunlight enters the water drop, refracts once, then reflects on the back of the water drop, and finally refracts again when it leaves the water drop. Because water has a dispersive effect on light, the refractive index of light with different wavelengths is different, and the refractive angle of blue light is greater than that of red light. Because light is reflected in water droplets, the spectrum seen by the observer is reversed, with red light at the top and other colors at the bottom.

The optical principle that causes rainbows often sees two rainbows appear at the same time, and a concentric but dark secondary rainbow (also called neon) appears outside the ordinary rainbow. The secondary rainbow is formed by two reflections of sunlight in water droplets. The strongest reflection angle of the two reflections occurs at 50 to 53, so the position of the secondary rainbow is outside the main rainbow. Because there are two reflections, the color sequence of the auxiliary rainbow is opposite to that of the main rainbow, with the outer side being blue and the inner side being red. In fact, the auxiliary rainbow must follow the main rainbow, but sometimes it is invisible to the naked eye because of the low light intensity.