Light travels in straight lines, so when something blocks it the light simply can't reach the floor behind — that dark patch is the shadow.
Most people think a fuzzy shadow edge means the object itself is blurry or moving. In fact the blur comes from the width of the light source: a wider source lets edge spots see part of it, creating the soft penumbra.
What's actually happening
Light is the straightest thing in the universe. It does not flow around obstacles the way water or sound does — if a straight line from the light source to the floor passes through your body, that bit of floor goes dark. A shadow is just the full map of all the blocked straight lines.
The interesting part is the edge. A truly point-sized light source draws razor-sharp shadows: every spot on the floor either sees the source or it doesn't. But real sources (the sun, a lamp, a window) have width. Spots near the shadow's edge can see part of the source, so they are only partly dark. That part-shadow fringe is the penumbra; the fully-dark core is the umbra. The bigger the light source looks from where the shadow falls, the softer and fuzzier the edge.
Solar eclipses are this exact picture at planetary scale. The Moon casts an umbra a few hundred kilometres wide that races across Earth — stand inside it and the sun is fully blocked; stand in the vast penumbra around it and you get a partial eclipse. Same physics as your hand over a lamp, scaled up a billion times.
A shadow is just the map of blocked straight-line rays; the size of the light source, not the object, decides how sharp or soft its edge looks.
- 1In a dark room, hold your hand a few centimetres above a table and light it with a phone torch from high up — the shadow has crisp edges.
- 2Now tape a sheet of baking paper just in front of the torch to spread the light. The same hand now casts a soft, fuzzy-edged shadow.
- 3Move your hand toward and away from the table and watch the fuzzy penumbra grow and shrink. You are resizing the light source as seen from the shadow.
Common questions
A tiny, point-like source gives a crisp shadow with a fully dark umbra. A wider source lets spots near the edge see part of it, creating a soft, partly lit fringe called the penumbra.
An overcast sky acts as one enormous light source. The penumbra grows until it swallows the umbra entirely, because light arrives from everywhere and nowhere is fully blocked.
Each separate light source draws its own shadow in its own direction. Under stadium floodlights an athlete can cast four shadows, one for each tower.