Teaching article
Shadow size changes when the object moves
Change the object's position while keeping the light and screen fixed.
Fix the torch and screen, then mark several positions for the card shape. Tracing the projected outlines gives children evidence to compare, while the unchanged components make the altered distance easier to identify.
To hand: A torch, card shape and screen.
- Mark the fixed positions.
- Move the card.
- Compare shadow outlines.
Keep two parts fixed while moving one
Moving the card nearer the torch can enlarge its shadow.
Trace the projected outline at each marked position. Avoid relying on memory when comparing shadows produced at different moments.
Which distance changed? Use a partner to position the card.
Do not move every component together.
Draw the arrangement before changing it
Place a stable torch facing a fixed pale screen. Between them, support a simple card shape upright. Mark all three positions on the table, leaving room for the card to move along a short line. Keep the beam below eye level and the working area clear. A large plain shape makes changes easier to see than a detailed silhouette.
Ask children which parts must stay still to investigate the card's position. Label the torch and screen as fixed. Record the initial card position with a letter rather than relying on a child's memory. The card should also keep the same orientation: turning it while moving it would change the comparison.
Preserve each outline
Trace or photograph the shadow at position A. Move only the card toward the torch and repeat at B. Use different colours or separate labelled sheets. Ask which shadow extends farther vertically or horizontally, then measure a chosen dimension if appropriate. The choice of dimension matters when an outline is irregular; “bigger” can otherwise mean different things to different observers.
A card closer to a small light source can block a wider portion of the light reaching the screen, producing a larger projected shadow in this arrangement. A simple side-view drawing can show straight lines from the torch past the card's edges to the screen. Present this as an explanation of the model, with the torch treated as a small source, rather than a promise that every lamp produces equally sharp boundaries.
If the shadow becomes blurred, record the fuzzy edge rather than tracing a falsely exact line. Ask children how confidently they can compare the sizes. Moving the screen would create another useful investigation, but it needs its own plan because it changes a different distance.
Children can direct a partner to position the card, choose the matching outline or compare prepared photographs. Offer tactile card shapes before the demonstration and oral descriptions of the projected change. A final check can show a diagram where every component has moved. Ask why that record makes it harder to attribute the size change to the card's position alone.
Resources
- Torch investigation – Light and Shadow Major Activity — Follow the torch activity when investigating projected light and shadows.
Further reading
Read next
- Move the torch and follow the shadow's direction
- Not every metal-looking sample is attracted to a magnet
- Weather now and weather forecast: read the time reference
Explore Forces, light and weather. Browse all teaching articles.