Ideas to emphasise here

• Construct the source-medium-detector model
• Show illumination as a process where something travels
• Build a helpful model of how illumination diminishes
• Emphasise that there is a finite trip time from source to detector
• Draw out the role of light in enabling seeing
• Bring to mind the role of reflection in seeing luminous objects
• Always explicitly accounting for reductions in brightness
• The physical aspect of the transmission of light
• Light is emitted by luminous objects
• Light travels in straight lines
• Objects are transparent, translucent or opaque
• Objects that are opaque cast shadows
• We only see non-luminous objects when light bounces off them
• Sometimes, if the object is shiny, this bouncing can form an image (mirrors)
• Light is detected by our eyes
• Light travelling is the spreading of the vibrations
• Link reductions in intensity with distance from the source
• Link delays in hearing sounds compared to seeing sights to the trip time of propagation from the source
• That light travels through a vacuum (from the Sun and other stars)

Teacher Tip: Work through the Physics Narratives to find these lines of thinking worked out and then look in the Classroom Activities for some examples of activities.

Strategies for supporting learning

• Identify source and detector
• Separate luminous from non-luminous
• Draw out children's ideas about seeing
• Connect seeing to the source-medium-detector model
• Build an explicit model of seeing
• Organise what children have observed into a coherent whole
• Build three-dimension models where illumination is important
• Draw out what children believe about the Earth-Moon-Sun system and how this is related to everyday phenomena
• Avoid restricting the idea of reflection to shiny surfaces
• Reinforce the role of reflection in seeing
• Connect seeing to both specular and diffuse reflections
• Show clear examples of the phenomena
• Introduce a wide range of surfaces from which reflection happens
• Put the source–medium-detector model to use; note the lack of tangible medium
• Connect light sources (luminous objects) to how we detect them
• Connect seeing to the source-medium-detector model
• Separate the luminous object that generates the vibration from the propagation of the vibration, which is also a movement
• Emphasising that all lights have a source
• Emphasising that we only see non-luminous objects when light bounces off them
• Tracing the chain from source to detector, via object, often
• Connect human vision to what other species can see (for example snakes can see in IR and insects in UV)
• Link each light seen back to the source, via the object it bounces off
• Look at different early models of seeing to see which ones fit the evidence of our experiments.

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Avoid these

• Speaking or acting as if light was just there
• Assuming that how we see is well understood
• Assuming that seeing is all accounted for by the physics
• Conflating rays (the theoretical construct) with light beams (the physical)
• Running together what is noticed and recorded with what is modelled
• Restricting examples of reflection to only, or mostly, shiny surfaces
• Replacing experiences of real and interesting phenomena with a series of ad hoc memorised rules
• Not showing how an understanding of simple situations is linked to a wide variety of phenomena in the lived-in world
• Introducing the technical term 'ray'
• Using specious energy descriptions
• Drawing or showing transverse waveforms
• Asserting that light is a wave without clarifying explanation of the idea of a wave – this is hard
• Introducing wavelength, frequency, or energy of light

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.