Introducing the light postulate

The light postulate is a central concept of Special Relativity Theory and notoriously difficult for students to master [1,2]. The postulate states that ‘light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body’ [3]. Prior to instruction, secondary students reason with a speed of light that is constant either to the light source or some form of absolute space [2]. These student models show parallels to two ideas on the nature of light from the history of physics: Newton’s idea of light as tiny particles and Huygens who described light as waves [4]. Drawing on the model of educational reconstruction, we have designed a teaching and learning sequence (TLS) that builds on student’s pre-instructional light propagation models and ideas from the history of physics. The TLS allows students to explore and evaluate these models. This provides students with the tools to propose a new light propagation model, the light postulate, themselves. Results with small groups of students, show that students indeed propose the light postulate and use this model to make predictions in relativistic context’s and use the light postulate to derive relativistic concepts such as time dilation and length contraction [4]. References [1] Villani, A., & Pacca, J. L. A. (1987). Students’ spontaneous ideas about the speed of light. IJSE, 9(1), 55-66. [2] Kamphorst, F., Vollebregt, M. J., Savelsbergh, E. R., & van Joolingen, W. R. (2019). Students’ pre-instructional reasoning with the speed of light in relativistic situation. PRPER, (15), 020123. [3] Einstein, A. (1905). On the electrodynamics of moving bodies. Annalen der Physik, 17. [4] Kamphorst, F., Vollebregt, M. J., Savelsbergh, E. R., & van Joolingen, W. R. (under review). An Educational Reconstruction of Special Relativity for Secondary Education.