cut a part of a tracing paper with scissors and stick it to one of the external walls of a vessel. to the opposite wall stick a diffraction grating. the higher number of slits per 1mm in a grating, the bigher is the angle at which an interference pattern will be visible (one can have doubts about a possibility of comparing values of sinus and tangent of the angle at which the pattern is visible). nonetheless, the obtained pattern shall be sharper, so defining its location will be easier.