Light & shadow

Casting shadow animals on the wall with a torch in the evening, seeing your own short shadow in the schoolyard at noon: shadows follow you wherever there is light. How they form and why they are sometimes huge and sometimes tiny becomes clear with one single basic idea about light.

Light sources and lit objects

Some things produce light themselves: the sun, a lamp, a candle, or a screen. They are called light sources. Most things around you do not glow on their own though, you only see them because they reflect light. The moon is such a lit object: it looks bright, but it only reflects the sunlight that falls on it.

Straight lines, that's why shadows exist

Light travels in straight lines, it cannot bend around corners. If an object is in the way, no light arrives behind it: that is where its shadow forms. How big the shadow gets depends on the distances between lamp, object and wall. Imagine your hand and the wall stay where they are and you move only the lamp. Move the lamp closer to the hand and the hand blocks a larger part of the light, so the shadow on the wall grows. Pull the lamp back again and the shadow shrinks.

Nothing is faster than light

Light races through the world at about 300,000 km per second, the highest speed there is. Sound in air only manages around 340 m per second. That is why in a thunderstorm you see the lightning practically instantly and hear the thunder only seconds later. Even the light of the sun, by the way, travels a good 8 minutes to reach us.