Oscillations in V(z) curves of thin samples

The explanation of the oscillations appearing at large defocus in the V(z) curves of thin specimens is given for isotropic and cubic materials. Using a ray model it is shown that oscillations are generated by the interference of the normal ray with the shear ray reflected from the bottom surface along a symmetrical path to the lens. The model predicts that in isotropic materials oscillations arise at critical values of defocus, while in anisotropic solids the onset of the oscillations depends on the shape of the slowness surface. In particular, for cubic crystals cut along.<>