Measuring How Shadow Size Changes with Light Distance
About this lesson
In this 45-minute Grade 3 investigation, pairs cut out a Halloween shape, shine a flashlight on it from 20, 40, and 60 cm away, and trace and measure the shadow each time. Children record their results in a table and use the numbers to explain why a closer light makes a bigger shadow. You'll need flashlights (one per pair), paper, scissors, rulers, masking tape, and science journals.
- Grade
- 3
- Subject
- Science
- Time
- 45 min
Objective
Students will investigate how moving a light source closer to or farther from an object changes the size of its shadow, and explain the pattern they observe using their recorded measurements.
Materials
- Flashlights (one per pair)
- Paper, pencils, and scissors for cutting simple Halloween shapes such as a bat, ghost, or cat
- Rulers or metre sticks
- Masking tape
- Science journals or lined paper
Hook
Dim the classroom lights. Quietly hold a paper ghost cutout in front of a flashlight so its shadow appears on the wall. Ask students to sit still, watch silently for one minute while you slowly move the flashlight closer and farther away, and then write one sentence in their journals describing what they noticed.
Main Activity
In pairs, students cut out a simple Halloween shape about 10 cm tall and tape a sheet of paper to the wall or the side of a desk to act as a shadow screen. One partner holds the cutout 10 cm in front of the screen and keeps it still while the other partner shines the flashlight from 20 cm, 40 cm, and 60 cm away from the cutout, measuring each distance with a ruler. At each distance, the pair traces the outline of the shadow on the screen paper and then measures the shadow's height in centimetres. Students record their results in a three-row table in their journals with the columns 'Flashlight distance (cm)' and 'Shadow height (cm)', then switch roles and repeat the test once to check their results. Ask students to work in calm, quiet voices, as careful scientists would. Close with five minutes of independent journaling in which each student writes the pattern they found and draws a labelled diagram showing the light, the object, and the shadow.
Discussion Questions
- What happened to the shadow's size as the flashlight moved closer to the cutout, and why do you think that happens?
- Did your two trials give exactly the same measurements? What might explain any differences?
- Why do you think shadows can look larger and spookier on Halloween night when someone holds a light close to an object?
- Where else in your life have you noticed shadows changing size or shape?
- If you wanted to make the smallest possible shadow, where would you put the light, and why?
Exit Ticket
Complete this sentence and add one measurement from your table as evidence: 'When the light moves closer to an object, the shadow gets ______ because ______.'
Differentiation
Support: Before students begin, mark the 20 cm, 40 cm, and 60 cm distances on the desk with masking tape so they do not have to measure the flashlight position each time. Give struggling learners a pre-drawn data table and the sentence frame 'At ___ cm, my shadow was ___ cm tall.' Pair them with a supportive partner who can model careful tracing and measuring.
Extension: Advanced learners add two more distances (for example, 30 cm and 50 cm) and create a simple bar graph of shadow height for each flashlight distance. They then predict the shadow height at 80 cm, test their prediction, and write a short reflection on how close their prediction was and what they learned from it.
