Fig.1.

Illustration of ray interaction with a sphere. An ultrasonic transducer having a Gaussian beam profile is focused at F (a dot on the beam axis), and produces two representative rays denoted as a and b, along with its scattered rays around the sphere. Note that the thicker arrow indicates a ray carrying the higher energy. As the incident rays propagate through the sphere, due to the momentum transfer, the outgoing rays induce the pressures of P_a (a thick arrow) and P_b (a thin arrow), respectively. Because P_a is higher than P_b, the net pressure P will direct the sphere towards the focus F.