The interference of two crossed laser beams results in a standing wave. Such fringe patterns are exploited in different instruments such as interferometers or laser-Doppler anemometers. We create a fringe pattern in the sample plane of a microscope using a compact apparatus based on a Kösters prism. The fringe pattern is shown to be spatially and temporally very stable, covers a large area, and its spacing is easily tunable. In addition, we exploit it to impose a sinusoidal potential on colloidal particles.