4 (a) (i) Explain why ultrasound used in medical diagnosis is emitted in pulses. (ii) Explain the principles of the detection of ultrasound waves used in medical diagnosis. (b) The specific acoustic impedances Z of some media are given in Table 4.1. [Table 4.1]

1. (a)(i)Explain why ultrasound used in medical diagnosis is emitted in pulses.[2]
2. (a)(ii)Explain the principles of the detection of ultrasound waves used in medical diagnosis.[3]
3. (b)(i)The specific acoustic impedances of two media are Z₁ and Z₂. The intensity reflection coefficient α for the boundary of these two media is given by: α = (Z₁ - Z₂)² / (Z₁ + Z₂)² Calculate, to three significant figures, the fraction of the ultrasound intensity that is reflected at a boundary between air and soft tissue. α = ............................................................................ [1][1]
4. (b)(ii)Use your value in (b)(i) to explain why gel is applied to the surface of the skin during an ultrasound scan.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.