Flows through a slot were optically examined by Mach-Zehnder interferometer in a range of low Reynolds numbers. Fluids examined in this study are the so-called Newtonian fluids: ion exchange water, silicone oils and liquid paraffin. It was found that interference fringes of laser lights are anomalously deformed around the slot. The deformation was small but non-zero even for ion exchange water. It was getting larger and changing with time for silicone oils and liquid paraffin. Moreover, some bright lines appeared around the slot. Also it was found that there is some region around the slot where the refractive index is higher than that in the surrounding region. The flow field was visualized using reflex powders and a laser light sheet, and found to be a normal converging and diverging flows expected for the low Reynolds number realized in the present experiment. These facts mean that the region around the slot is apparently in lower temperature and it is very hard to explain it from the heat generation by the flow around the slot.

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