A doctor uses ultrasound to look at structures inside a patient's body.
A probe placed on the skin sends pulses of ultrasound into the body. At each boundary between different tissues, some of the ultrasound is reflected and some is transmitted.
A layer of gel is spread between the probe and the skin.
The speed of ultrasound in soft tissue is 1540 m/s.
An echo from the boundary of an organ is detected 65 μs65\ \mu\text{s}65 μs after the pulse is sent.
Calculate the depth of the boundary below the skin.
Give your answer in cm\text{cm}cm.
Without the gel, there would be a thin layer of air between the probe and the skin. Almost all of the ultrasound would be reflected at the boundary between the air and the skin.
Explain why the gel is needed.
The echo from a boundary deep inside the body is much weaker than the echo from a boundary close to the skin.
Explain why.
52 exam-style questions on OCR GCSE Physics 5.1 Wave behaviour, covering 5.1.1 Describing wave motion, 5.1.2 Wavelength and frequency, 5.1.3 Wave speed, frequency and wavelength, 5.1.4 Wave speed problems, 5.1.5 Transverse and longitudinal waves, 5.1.6 Sound waves crossing between media, 5.1.7 Reflection, transmission and absorption at interfaces, 5.1.8 Sound waves and vibrations in solids, 5.1.9 Frequency range and human hearing, 5.1.10 Modelling waves with water ripples and sound, and 5.1.11 Evidence that the wave travels, not the medium. Each one has a worked solution and a mark scheme showing where the marks go.