Surface waves on cylindrical rods with dimensions much smaller than the incident radiation experience reflection from the ends of the rods thus effectively forming a cavity. Rods with flat ends and rounded ends have been studied in the past for determination of the phase of reflection, which plays a vital role in determining the resonant frequency. In this paper we present a simpler approach to evaluate the phase of reflection from rounded end rods by considering the limiting case of a sphere. We approximate the extreme sub-wavelength sphere by a Fabry Perot (FP) resonator to derive the relation for phase of reflection which will prove useful for the design of nanorods for various applications. Optical nanorods are extensively utilized in optical antennas, optical/biological sensors, light emitting devices and other optoelectronic devices.