(a)[2]
State what is meant by acoustic impedance, with reference to ultrasound waves.
(b)[3]
An ultrasound wave is incident on the boundary between two media. The acoustic impedances of the two media are $Z_1$ and $Z_2$, as shown in Fig. 10.1. Explain the importance of the difference between $Z_1$ and $Z_2$ for the transmission of ultrasound across the boundary.
(c)[2]
Ultrasound frequencies as high as $10\,\text{MHz}$ are used in medical diagnosis. State and explain one advantage of high-frequency ultrasound compared with lower-frequency ultrasound.