An autonomous inspection drone uses ultrasonic echolocation (sonar) to measure the clearance to the bottom of a deep vertical mine shaft.
The drone emits short pulses of sound waves vertically downwards. Some of these waves reflect off the floor of the shaft and are detected by a sensor on the drone.
The speed of sound in the air inside the shaft is 0.344 km/s0.344\text{ km/s}0.344 km/s. The shortest time interval between a pulse being emitted and its echo being detected is 0.25 s0.25\text{ s}0.25 s.
Calculate the distance from the drone to the nearest point on the floor of the shaft.
Each ultrasonic pulse is emitted for a very short duration.
The reflected echo pulse detected by the drone's sensor is found to have a significantly longer duration.
Suggest two reasons for this difference in duration.