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Science and Technology K–6 · Year 5

Shadow size: moving the object toward the torch

Science understanding: Physical sciences

Practical, model not builtLow risk

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The idea

A shadow grows when the object moves closer to a small light source and shrinks when it moves toward the screen, because the light travels in straight lines that spread out from the source.

What you need

  • a small bright torch (a single LED, the nearer to a point source the better)
  • a card cut-out 5.0 cm tall on a stick
  • a white wall or screen 60 cm from the torch
  • tape measure and ruler
  • a darkened room

How to do it

  1. Fix the torch on a pile of books 60 cm from the screen and switch it on.
  2. Hold the 5 cm cut-out 20 cm from the torch and measure the height of its shadow on the screen. Record it.
  3. Move the cut-out to 30, 40 and 50 cm from the torch and measure the shadow each time.
  4. Graph shadow height against object distance.
  5. Replace the torch with a wide fluorescent lamp or a torch with a large diffuse lens and describe the shadow's edge.
  6. State the rule linking shadow height, object height and the two distances.

What you should see

Shadow height = object height x (screen distance from the torch / object distance from the torch): at 20 cm the 5 cm cut-out throws a 15 cm shadow, at 30 cm a 10 cm shadow, at 40 cm 7.5 cm and at 50 cm 6 cm, tending toward 5 cm as the object reaches the screen. A wide light source gives a blurred edge (a penumbra) because different parts of the lamp cast shadows in slightly different places.

What changes

What you change
distance from torch to object
What you measure
shadow height
What you keep the same
  • object height
  • torch-to-screen distance
  • torch position

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Shadows are the same size as the object.
  • A shadow gets bigger when the light is brighter.
  • Moving the object toward the wall makes the shadow bigger.

Safety card

Low riskLearners carry it out

Hazards

  • torch in eyes

Controls

  • torch aimed at the screen only

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Science and Technology K-6 Syllabus (2017), current, taught until 2026; a Stage 2 outcome, because NSW teaches the behaviour of light in Stage 2 ('investigate the behaviour of light', with reflection in a mirror and shadows as its examples), and neither the 2017 nor the 2024 Stage 3 content includes it; code read from the syllabus document on 22 September 2026ST2-8PW-ST
  • Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-1WS-S
  • Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-PQU-01ST3-DAT-01
  • Australian Curriculum v9AC9S5U03AC9S5I02AC9S5I03AC9S5I04

Sources

The pages the author read to write this activity.

  1. www.scootle.edu.au/ec/search?accContentId=AC9S5U03
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  4. www.questacon.edu.au/learn-and-play/activities/changing-shadows
  5. primaryconnections.org.au/v84-sequences/light-shows

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