If you see a rainbow from a plane - at an altitude of 40000 ft - what shape is it... a circle or a straight line or as you would see it from the ground?

When viewed from a plane at an altitude of 40,000 feet, a rainbow appears as a circular arc rather than the typical semi-circular shape seen from the ground. This is because rainbows are formed by the refraction, reflection, and dispersion of light in water droplets, creating a full circle of colors when observed from a high vantage point. However, due to the horizon obstructing the bottom half, only the upper arc is usually visible.