Shadows and how they change
Physics · Key Stage 2 · The School
Blocked light
A shadow appears where an object blocks light. It needs three things in a line: a light source, an OPAQUE object that light cannot pass through, and a surface for the shadow to fall on. Transparent things such as clear glass make almost no shadow; translucent things such as tracing paper make a faint one, because some light gets through.
Nearer the light, bigger the shadow
Move the object towards the light and its shadow grows; move it away and the shadow shrinks. This is because a nearer object blocks a wider spread of the rays leaving the source. You can test it with a torch and a hand in seconds, and predicting the direction of the change before you try it is the useful part.
Shadows through the day
The Sun appears low in the sky in the morning, higher around midday, and low again in the evening — because the Earth is turning. A low Sun casts long shadows and a high Sun casts short ones, so a shadow is longest at the start and end of the day and shortest in the middle. It also swings round, which is how a sundial tells the time.
Stand a stick upright in a patch of ground outside on a sunny day. Mark the tip of its shadow at 9am, noon and 3pm. At 9am the shadow is long and points one way; at noon it is at its shortest; by 3pm it is long again and points the other way. Three marks, and they show both things at once — the Sun's apparent height changing, and the direction sweeping round as the Earth turns.