Light in a straight line
Why Do Shadows Happen?
Light cannot bend around you. Where it is blocked, there is a dark shape.
-
Step 1 of 5
Light only goes in straight lines.
A beam never curves round a corner on its own. This one fact explains every shadow you have ever seen.
-
Step 2 of 5
An object blocks the rays that hit it.
The light heading toward your hand stops at your hand. The light passing either side keeps going and lands on the wall.
-
Step 3 of 5
Closer to the light means a bigger shadow.
The rays spread out as they travel. Move your hand near the lamp and it blocks a wider cone, so the dark patch on the wall grows enormous.
-
Step 4 of 5
Big lights make soft edges.
Cloud turns the whole sky into one enormous lamp, which is why an overcast day has no shadows at all. A tiny bulb gives razor-sharp shadows. A large window lights your hand from many angles at once, so the edge fades gradually into a fuzzy border.
-
Step 5 of 5
The shadow's shape is a flat squash of the object.
A shadow has no depth — it is the outline of whatever blocked the light. Turn the object and the outline changes completely.
Sunlight comes from so far away that its rays arrive parallel — which is why sun shadows are the same size as the object.
The short version
A shadow is the straight-line gap left where an object stopped the light.
Try it yourself
Hold your hand between a torch and a wall. Move it slowly toward the torch and watch the shadow swell.