We do not see the shadow on the ground of a bird flying high in the sky. However, the shadow is seen on the ground when the bird swoops near the ground. Think and explain why it is so.
When a bird flies high in the sky, its shadow is not visible on the ground because the Sun is not a point source of light but an extended source.
Light rays from the Sun reach the bird from different angles. When the bird is at a high altitude, the umbra (the completely dark region where no light reaches) does not extend all the way to the ground.
Only the penumbra (the partially shaded region) reaches the ground, which is very faint and spread over a large area, making it difficult to see.
When the bird flies closer to the ground, the umbra region reaches the ground, creating a visible dark shadow. The shadow becomes more defined and concentrated in a smaller area, making it clearly visible.
Explanation
This question relates to shadow formation concepts from section 11.4. The chapter explains that shadows are formed when an opaque object blocks light. However, the nature of the shadow depends on the distance between the object and the screen (ground in this case). The Sun acts as an extended source of light, not a point source. When an object is far from the screen, the umbra (dark shadow) becomes very small or may not even reach the screen, while the penumbra (lighter shadow) spreads over a large area and becomes very faint. When the object is close to the screen, the umbra is larger and more defined, creating a visible shadow.
Solution Steps
The Sun is an extended source of light, not a point source, so light rays reach objects from different angles.
When a bird is at high altitude, the umbra (dark shadow region) does not reach the ground.
Only the penumbra (partially shaded region) reaches the ground when the bird is high up, which is faint and spread over a large area.
When the bird flies closer to the ground, the umbra reaches the ground, creating a defined and visible shadow.