I am sorry, this sentence just doesn't make sense to me:
REQUIREMENT: the device should have a transmitter and a receiver inorder to continously transmit and receive the REFLECTED radiation...
The ellipsis at the end adds no useful information. It seems you are trying to determine the water level in a beaker so when that beaker gets full, the water flow can be turned off, and similarly, when the level reaches a pre-determined low mark, the water source can be turned on again.
Why do you want to continuously transmit and receive the reflected "radiation?" To where are you transmitting the reflected radiation?
Ultrasonic distance measuring has been suggested. That seems reasonable to me. But, you seem to have rejected it. The method I suggested will also work, if the container is reflective. BTW, in the method I used, the depth is determined from the reflection off the bottom of the container, not the top surface of the water.
Can you post a sketch to help eliminate some of our confusion?
John