Overview:  The teacher introduces the problem to students as a problem about light and shadows.  The teacher recalls a model of light suggesting that light travels and straight lines and says that students will use this model to think about how to place a light source to make a shadow of a given size.  A student in the class, Evie, brings up the point that there is also a model of light where light travels in waves (as opposed to straight lines).  With this comment, students in the class become distracted talking about Physics, Einstein, and models of light.  The teacher takes this opportunity to lead a discussion about the debate between the wave model and the particle model of light.  Students become confused by the discussion, and the teacher returns to the idea that students will be working on a problem about puppets, and they will not need to use physics.

Prior knowledge:  The teacher attempts to draw upon students’ prior knowledge of the context of the problem, the context of physics and light.  Because students are currently taking geometry, it is not clear whether students in the class have studied physics, or how much prior knowledge students have of different models of light.  However, because a student brings up an alternative model to what the teacher originally has proposed, the teacher sees an opportunity to engage students in a discussion about different potential models for how light travels.  At the end of the discussion the teacher tells students, “you don’t have to think about physics when you do puppets,” so in a way the teacher tells students to disregard the knowledge that she has just elicited.

Other points of interest:  It seems that the teacher intended with this launch to introduce a model of light traveling in straight lines and then to provide students with materials to begin modeling the problem.  The launch evolves once Evie asks a question about the wave model of light.  The conversation in the classroom shifts from a teacher monologue to student questioning dialogue, where students ask questions and control the topics of the dialogue (Lemke, 1990).  The student questioning actually allows the teacher to explore the context of the problem more than she may have originally intended.