English: Foucault's determination of the relative speed of light in air vs water. Light from a passing through a slit (not shown) is reflected by mirror m (rotating clockwise around c) towards the concave spherical mirrors M and M'. Lens L forms images of the slit on the surfaces of the two concave mirrors. The light path from m to M is entirely through air, while the light path from m to M' is mostly through a water-filled tube T. Lens L' compensates for the effects of the water on the focus. The light reflected back from mirrors M and M' is diverted by a beam splitter towards an eyepiece O. If mirror m is stationary, both images of the slit reflected by M and M' reform at position α. If mirror m is rapidly rotating, light reflected from M forms an image of the slit at α' while light reflected from M' forms an image of the slit at α". The experiment demonstrated that light travels more slowly through water than air. Thus, it disproved Newton's hypothesis that Snell's Law results from an increase in the speed of ballistic light particles as they crossed an air/water interface, and gave what was considered to be definitive support of the wave theory of light.