The Smith-Purcell effect was the precursor of the free electron laser (FEL). In their experiment, Steve Smith, as a graduate student guided by Purcell, sent an energetic beam of electrons very closely parallel to the surface of a ruled optical diffraction grating, and thereby generated visible light. He showed there was negligible effect on the trajectory of the inducing electrons. Essentially, this is a form of Cherenkov radiation where the phase velocity of the light has been altered by the periodic grating..