The emitting face of an ultrasound transducer is in direct contact with the upper surface of the oil in a storage tank. It sends a pulse vertically downwards through a 0.52 m layer of oil and then a 0.90 m layer of water. The pulse travels at 1300 m/s in oil and 1500 m/s in water. Echoes are detected from the oil–water boundary and from the base of the tank.
Which pair gives the arrival times of these two echoes, measured from when the pulse is sent?
1.6 ms1.6\ \mathrm{ms}1.6 ms from the oil–water boundary; 2.8 ms2.8\ \mathrm{ms}2.8 ms from the base of the tank
0.80 ms0.80\ \mathrm{ms}0.80 ms from the oil–water boundary; 2.0 ms2.0\ \mathrm{ms}2.0 ms from the base of the tank
0.80 ms0.80\ \mathrm{ms}0.80 ms from the oil–water boundary; 1.2 ms1.2\ \mathrm{ms}1.2 ms from the base of the tank
0.40 ms0.40\ \mathrm{ms}0.40 ms from the oil–water boundary; 1.0 ms1.0\ \mathrm{ms}1.0 ms from the base of the tank