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.
Light height is how high we hold the torch above the table. That is the one thing we will change today.
A fair test means we change only one thing, measure one thing, and keep everything else the same. If we keep the object still and only move the torch higher or lower, what do you think will happen to the 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?
Why does the shadow change?
Our question: does torch height change how long a shadow is? Five cards appear on screen: torch height, shadow length, the object, where the object sits, and the 30 cm horizontal distance from torch to object. Call out which bin each card belongs in: Change, Measure, or Keep the same.
Then, as a group, choose the three torch heights you will test (for example 10 cm, 20 cm and 30 cm above the table, or three clear heights of your own). Write those heights on your Investigation Journal page so you are ready for the test.
Does torch height change shadow length?
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.
Your Investigation Journal page has the two columns ready: torch height (cm) and shadow length (cm).
- Fix the object on its tape mark.
- Set the torch 30 cm away horizontally (ruler flat from object base to the point under the torch).
- Test the three torch heights your group chose (for example 10 cm, 20 cm and 30 cm above the table).
- Each time, measure the shadow length in centimetres from the object base to the tip of the shadow.
- Write each torch height and shadow length on your Investigation Journal page as you go — do not wait until the end.
- Keep the object, its place and the 30 cm horizontal distance the same every trial. Take each height twice if you can and check the readings match.
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