This image was acquired from
wikimedia. It was marked as Public Domain or CC0 and is free to use. To verify, go to the source and check the information there.
Keywords from Image Description:
Foucault's determination of relative speed of light air vs water. en Foucault's determination of the relative speed of light in air vs water Light from passing through slit not shown is reflected by mirror rotating clockwise around towards the concave spherical mirrors and M' Lens forms images of the slit on the surfaces of the two