Question

when a fan is rotating we observe the blades are rotating in opposite direction. Why this is happening?

Answer 1

The blades of the fan are not perfectly plane. They are bent in a specific direction. They are also configured to rotate in a specific way(clockwise or anticlockwise). Both of these factors contribute to the fact that a fan always gives windflow in a specific direction(the direction to which the fan is pointed at).

If any one of the two factors are reversed, the direction of airflow will get reversed. Reversal of both of these factors will not change the direction of airflow. That is, if you change the configuration of the blade-curve, or change the polarity of the current in the fan circuit,(thus changing the direction of rotation), the fan will give airflow in the opposite direction of which it was meant to do so