The sun is much farther away from us, compared to the lamp.
Work Step by Step
The sun puts out spherical wavefronts just as the nearby lamp does (see Figure 29.3), but we are so far away that by the time it gets to us, the expanding spherical wave can be considered to be a plane wave. In an analogous way, a sufficiently small area of Earth's spherical surface can be considered to be flat. The lamp is close enough that the curvature of its emitted wavefronts cannot be ignored.