Hi Vignesh,
Connect the rotating shaft to a flywheel whose tooths have size larger than an LED. Arrange the flywheel in between an IR LED and an IR photodiode. Now as the flywheel rotates the tooth will block and let the light fall on the IR photodiode continuously. Hence the output of the photodiode will be a pulse. Just count the number of pulse generated for a predefined time and then calculate the RPM from it. You can also measure the pulse width and with the help of which you can calculate the instantaneous RPM.