Focusing Schlieren technique is used to measure the span-wise length of the bifurcated normal shock in a rectangular duct. This is done by inclining the Schlieren setup by a small angle to the regular schlieren light path. The projection of the normal plane of the shock can be seen in the images, and can be used to deduce the spanwise length of the shock. The measurement is further verified by traversing the focusing schlieren setup to mark the physical distance between the ends of the normal part of the shock. This is the first reported work of measurement of spanwise length of a normal shock in a duct, and its temporal variation. © 2018 Elsevier Inc.