Answer: Tthe operation of transistor as a switch followed by the steps below

 1.As long as $V_i$ is low and unable to forward bias the transistor, $V_o$ is high at $V_{CC}$

1. If $V_i$ Is high enough to drive the transistor into saturation then $V_o$ is low, very near to zero. 

2. When the transistor is not conducting it is said to be switched off and when it is driven into saturation it is said to be switched on.

4.We can say that a low input to the transistor gives a high output and high input gives a low output.

1. When the transistor is used in the saturation state it acts as a switch. 

Below is the circuit diagram which show that transistor can be used as switch