What is a shadow?

On a sunny day, a dark shape copies your every move on the ground. Wave, and it waves back. It is your shadow — and it appears for a surprisingly simple reason.

Light likes to go straight

Light travels in straight lines, zooming forward until something gets in its way. Most things, like your body or a tree, are solid. Light cannot pass through them.

So when you stand in the sunshine, the light reaches all the ground around you — but it cannot reach the patch right behind you, because you are blocking it. That dark, light-free patch is your shadow!

A shadow is just missing light

A shadow is not really a thing you can hold. It is simply a place where the light has been blocked. That is why your shadow is always the same shape as you, and always falls on the side away from the light.

Long shadows, short shadows

Watch how your shadow changes. When the sun is low, in the early morning or evening, it shines almost sideways, so your shadow stretches out long and thin. When the sun is high at midday, it shines down from above and your shadow shrinks into a tiny puddle at your feet.

Wonder fact: Sundials use this trick to tell the time. As the sun moves across the sky, a shadow swings around like a clock hand, pointing to the hour.