Answer
The concept is explained in Figure 26.13.
Work Step by Step
The sun has a finite extent and is not a point source of light. When the plane is flying at a low altitude, its shadow is sharp because light coming from different parts of the sun doesn't get a chance to spread out much behind it before it hits the Earth.
The shadow of a high-flying plane is filled in mainly by light tapering in from different parts of the Sun. This is the same effect responsible for the penumbra of a solar eclipse (page 495, Figure 26.15).
