Why Do Shadows Have Edges?

Light travels in straight lines, so shadows form sharp edges where the light gets blocked.

Imagine throwing a tennis ball at a wall. The ball flies in a straight path until it hits something. Light does the same thing. When a bright lamp shines on a toy car, the car stops the light from passing through it. The space behind the car stays dark because no light reaches there. This dark shape is the shadow.

Why is the edge so sharp? Think about a cookie cutter. When you press it into dough, the dough outside the cutter stays flat and the dough inside gets pushed down. The line between them is very clear. Light works like that cookie cutter. It goes around the object in straight lines. Where the light stops, the shadow begins. If the light source is small and far away, like the sun, the edges stay crisp. If the light is big and close, like a soft desk lamp, the edges get fuzzy because light sneaks in from the sides.

Just like your hand makes a distinct shape against the window, objects block light to create those dark outlines we call shadows.