  We present the experimental demonstration of Rydberg blockade in thermal
atomic vapor where the atoms are not necessarily be frozen. We show that not
all the interacting atoms but only the atoms with same velocity collectively
participate in the blockade process. Using this observation, we formulated a
suitable model based on super atom picture to study blockade interaction in
thermal vapor. We performed an experiment to measure Rydberg population in
rubidium thermal vapor using optical heterodyne detection technique and density
dependent suppression of Rydberg population is observed in suitable
experimental parameter regime. Further analysis of the experimental data using
the model verifies the scaling law for van der Waals interaction strength
$(C_{6})$ with principal quantum number of the Rydberg state with $11\%$ error.
Our result suggests multi-photon excitation in thermal vapor with suitable
laser configuration to probe Rydberg blockade interaction based optical
nonlinearity and many body effects.
