STEM is about the integration of science, technology, engineering and mathematics. To teach STEM effectively, students need practical examples of subject integration. A good example is the use of an electron microscope in teaching physics and biology. An electron microscope is an instrument in which the operation depends on electromagnetism and quantum mechanics, in particular wave-particle duality. In teaching physics, an electron microscope can be used to measure the separation of repeated structures that give rise to interference phenomena. This paper describes the use a tabletop scanning electron microscope to teach physics and biology together through imaging butterfly wings. Students are able to identify structures with a spacing comparable to visible light on butterflies exhibiting structural colour and much larger spacing on wings without structural colour.