Watch this shadow on the wall. When the torch creeps up or down, the dark shape stretches and shrinks. What do you think makes it longer or shorter?

A shadow is the dark shape you get when something blocks light. Opaque means light cannot go through it, so a wooden block is opaque and makes a clear shadow.

On the screen you will see a lamp, an object and the shadow it casts. Watch what happens to the shadow when the lamp moves higher or lower. Predict first: will the shadow get longer or shorter when the lamp is low?

Our question: does torch height change how long a shadow is? Take each card on the board in turn and call out the bin it belongs in — Change, Measure, or Keep the same — and say why you put it there.

Share the jobs before you start:

• Torch holder — holds the torch at the chosen height, measured up from the table with a ruler.
• Measurer — checks the 30 cm distance along the table and measures the shadow.
• Recorder — writes on the Investigation Journal page.
• Anyone else checks the 30 cm mark, the shadow tip or the height reading.

Start by fixing the object on its tape mark so it cannot drift, then set the torch 30 cm away horizontally, laying the ruler flat from the object's base to the point on the table under the torch. Now work through the three torch heights your group chose, for example 10 cm, 20 cm and 30 cm above the table.

At each height, measure the shadow in centimetres from the object base to the tip of the shadow, and write that torch height and shadow length on your Investigation Journal page straight away, because if you wait until the end you will not remember which length went with which height.

The object, its place and the 30 cm horizontal distance stay exactly the same for every trial — that is what keeps the test fair. If you have time, take each height twice and check that the two readings match.