Nationally and internationally there have been calls for a focus on STEM (science, technology, engineering and mathematics) teaching and learning in schools to prepare students for the many future careers in the STEM fields. One way to do this is through engineering activities that provide opportunities for integrating STEM to solve problems using engineering design principles. Based on the Australian curriculum developed by the Australian Curriculum and Assessment Reporting Association (ACARA), the researchers designed an optical engineering activity for Year Five students (9-10 year olds) that was a culminating STEM activity in a unit of work focussing on light. The task required the application of mathematics, science and technology concepts to build an optical instrument that would enable students to see a hidden object or person. The study found that using an engineering design model that involved building, testing and re-designing, afforded students opportunities to demonstrate the application of core mathematics, science and technology concepts. This article describes firstly, the foundational work on light and optical engineering that preceded the engineering problem, and secondly, the engineering problem.