total internal reflection fluorescence microscopy This technique probes the surface of fluorescently-labeled living cells with an evanescent wave generated by a light beam traveling between two media of differing refractive indices. An incident laser beam is reflected at a critical angle (total internal reflection) when it encounters the interface between a microscope glass slide and the aqueous medium containing the cells. Only fluorophores which are within a few nanometers of the surface are excited, while those farther away are not, making possible the study the interactions at the cell surface. fluorescence microscopy