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? 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.
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. Then finish the I predict... because... line: as the torch goes higher, will the shadow get longer or shorter, and why?
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.
Before you touch the torch, label two columns on your Investigation Journal page: torch height (cm) and shadow length (cm). Check the three heights your group chose are written there too.
Fix the object on its tape mark and set the torch 30 cm away along the table. Then work through your three torch heights.
At each height, measure the shadow from the object's base to its tip, in centimetres, and write the height and length on your Investigation Journal page straight away.
The object, its place and the 30 cm distance stay the same every time. If you have time, take each height twice.
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