The sun is considered an extended source of light because its size is not negligible compared to the distances over which its light is observed. Unlike a point source of light, which can be approximated as a single point from which light emanates, the sun has a finite diameter of approximately 1.4 million kilometers.

Due to its large size, different parts of the sun's surface (photosphere) are at different distances from an observer on Earth. This means that light rays from various points on the sun's surface travel different distances before reaching the observer. As a result, the light from the sun arrives at the observer with a range of optical path lengths, leading to differences in the wavefronts.

This extended nature of the sun's light source affects the formation of shadows. Instead of producing sharp and well-defined shadows like a point source, the sun creates shadows with a gradual change in brightness known as umbra and penumbra.

Therefore, the extended nature of the sun's light source has various implications for optical phenomena and can lead to interesting effects such as sunspots and limb darkening observed in solar observations.