A body having specific charge $8 \mu \mathrm { C } \mathrm { g } ^ { - 1 }$ is resting on a frictionless plane at a distance 10 cm from the wall (as shown in the figure). It starts moving towards the wall when a uniform electric field of $100 \mathrm {~V} \mathrm {~m} ^ { - 1 }$ is applied horizontally towards the wall. If the collision of the body with the wall is perfectly elastic, then the time period of the motion will be $\_\_\_\_$ s.