The shadow has suddenly grown, but three students have moved three different things: the torch, the card and the screen. There’s no clear way to tell which movement caused the change.
For your Year 5 investigation, mark the torch and screen positions and let only the card move. Students can then compare shadow heights and explain what changed. The activity focuses on straight-line light and shadow formation.
Fix the torch and screen first
You’ll need a small LED torch, prepared blunt-edged opaque card, a secured white screen, tape and a ruler. An adult should set up and secure the equipment. Keep walkways lit, don’t use lasers and never look into the beam.
Arrange the torch, card and screen in that order. Mark three card positions: A nearest the torch, B around the middle and C nearest the screen. Choose marks that keep the whole shadow visible.
Keep the card upright, at the same height and parallel to the screen. Use a spreading beam wide enough to reach past the card’s edges. A real torch may produce blurry edges, and a shadow that runs off the screen can’t be measured fully. Check these limits before students begin.
Predict before moving the card
Ask students to identify the light source, opaque object and screen. Which mark do they think will give the tallest shadow? Ask them to give a reason before testing.
The chosen variable is the card’s position. As it moves, its distances from both the torch and the screen change. Keep the torch, screen, card orientation and room lighting the same, and use the same card throughout.
Help students turn “Move it closer” into a clear plan: “Move the card from B to A while the torch and screen stay on their marks, then measure the shadow again.”
Measure the same part each time
Give group members roles as card mover, measurer and recorder. Agree on an edge rule—for example, measure vertically between the outer visible edges of the darker silhouette. Use the same unit and start each measurement at the ruler’s zero mark.
Record the torch-to-card distance and shadow height at A, B and C. Primary Connections’ lesson on making shadows shorter or taller explores this approach to measuring and comparing shadows.
| Position | Torch-to-card distance | Shadow height | Repeat height |
|---|---|---|---|
| A | |||
| B | |||
| C |
After C, return to A and enter a second shadow height in its Repeat height cell. Leave the other repeat cells blank unless you revisit those marks too. Does the starting arrangement give a similar measurement? It needn’t match perfectly.
Trace why the shadow changes
An opaque card blocks light from reaching part of the screen, leaving a darker region. With the torch and screen fixed, moving the aligned card towards the torch generally makes its shadow larger; moving it towards the screen generally makes it smaller. Questacon’s Shadow Puppets activity demonstrates this relationship.
Follow the straight boundary rays past the card’s upper and lower edges. Nearer the torch, the card blocks light across a wider span of the screen. Nearer the screen, that span is narrower. The centre ray stops at the opaque card.
Use the observations to decide
Try this conclusion frame: “At ___, the shadow height was ___. At ___, it was ___. We changed ___ and kept ___ fixed. These observations suggest ___.” Ask whether returning to A gave a similar height, and what evidence shows whether the card itself changed size.
If the pattern is unclear, check alignment, shifted equipment and edge choices. If a shadow ran off the screen, reset the marks and repeat the comparison. Keep the first notes rather than replacing observations with expected results.
Finish with one sketch
Sketch the torch, card and screen. Show which way you would move only the card to make its shadow larger, then use one observation from your table to explain your choice.
The shadow can change size while the card stays the same size and shape.