Earth science is the smallest discipline among the sciences in terms of number of certified teachers at the K-12 level (compared to biology, chemistry, and physics). Some states require no instruction in Earth sciences after middle school at all, yet it is becoming increasingly important for citizens to be prepared to make decisions about such topics as climate, energy, and land use. A sneakily legitimate way to ensure Earth science is taught in schools is for those who regularly conduct professional development for teachers to find cross-disciplinary activities that could be taught under the umbrella of another science discipline but that have a strong Earth science component.

For example, optics is a topic covered in a physical science or general science class, not necessarily in an Earth science class. However, having a good sense about how light waves are refracted and reflected at the interface between two materials has direct applications for grasping how seismic body waves travel through the mantle. Here I detail a hands-on lab activity created for teacher online professional development. In this activity, students/teachers calculate the index of refraction of water by measuring the angles of incidence and refraction of light as it passes from air to water. This lab is part of a larger lesson in which the ultimate objective is for students/teachers to make their own observations using real seismic data and be able to build a chain of inference to observe that body wave speeds increase with depth and follow curved paths. However, it may also be used as a standalone activity to teach Snell's Law.