Alternative conceptions about light and shadows

Children naturally and instinctively develop their own ideas about how things work. These self-developed concepts make sense to the individual but may be scientifically inaccurate. It is helpful to know some of the common alternative conceptions students may hold. Awareness helps educators identify them when they surface in discussions and provides an opportunity to scaffold change.

Common alternative conceptions about shadows

Students may hold some of the following alternative conceptions:

• A shadow is an actual object.
• A shadow is an exact copy of an object.
• A shadow comes from within a living thing and follows it around.
• The size of a shadow depends on the size of the object.
• Shadows only occur outside.
• Shadows only occur during the day.
• A shadow exists because the object absorbs the light.
• A shadow is a kind of reflection because it has the same shape as the object.

Forming accurate conceptions

It takes time to change alternative conceptions. Research shows that students can hold multiple conceptions – their own explanation and an explanation they have learned from another source, like school – at the same time. Just telling the student the correct answer will not lead to lasting change.

To form accurate conceptions, students must:

• understand the new information
• find it believable
• become dissatisfied with their prior thinking.

The two ways students do this are:

• assimilation – they gradually adjust their ways of thinking to include new information
• accommodation – they develop a new mental blueprint.

Engaging discussion and deepening understanding

While students are actively exploring light and shadows, use the opportunities to make predictions, ask questions and engage in discussion to:

• check prior knowledge about light and shadows
• check for alternative conceptions
• use observations (gather and interpret data) to challenge alternative conceptions
• identify scientifically plausible explanations
• consolidate or extend thinking
• develop content vocabulary
• encourage communication, comparison and analysis between individuals and groups
• look for opportunities for students to design simple investigations to answer questions.

Building science knowledge requires multiple experiences over time

Light is a complex topic. As a physical phenomenon, it is mentioned in all eight levels of the New Zealand Curriculum. There is recognition that students will build their science understanding about light (and shadows) over several years.

Concepts often build sequentially. The New Zealand Ministry of Education resource Building Science Concepts Book 9: Shadows: Effects of the Absence of Light lists the likely sequence:

• The shape of an object determines the shape of its shadow.
• An object is always between a light source and the surface on which its shadow forms.
• Light travels in straight lines outwards from its source.
• A shadow is the relative absence of light.
• The angle at which light strikes an object or surface determines the size and shape of the shadow.

The images on this page provide visual representations of each concept.

The student activity Investigating shadows provides multiple ideas for exploring these key concepts. Extend understanding of why shadows change during the day with the activity Investigating shadows and the position of the Sun. Explore how the properties of an object affects its shadow with the activity Investigating shadows using transparent, translucent and opaque materials.