A rainbow viewed from an airplane may form a complete circle. Where will the shadow of the airplane appear? Explain.

The position of the sun must be at the horizon in order for the rainbow to appear.

The relative positions of the sun and plane make the rainbow appear as a full circle.

However, if the sun is at the horizon, the shadow of the plane will be behind the plane, not below it; this means that the shadow of the plane will not fall on the earth.