Natural convection studies in an enclosure are very relevant since it spans a wide area of applications such as heat exchangers, chemical reactors, cooling of electronic equipment, etc. In the present study, an experimental investigation is performed in a rectangular cavity of dimension 46 mm x 23 mm with protruded copper half cylinders of diameter 15.8 mm (5/8 inch) each. Interferometry has always been a good technique for accurate measurements of temperature gradients. Mach-Zehnder interferometer is employed for visualising the isotherms and for quantitatively inferring the temperature gradient. The protruded half cylinders are maintained at constant temperature, ranging from 60°C to 120°C using an electric cartridge heater and associated temperature controller setup. A He-Ne laser with 24mm collimated beam size is used for experimental purposes. Temperature gradient variation along the radial direction is plotted against the circumferential angle of the cylinder and isotherm merging angle is determined for varying time step and varying cylinder surface temperature.