A technician uses ultrasound to identify a stone sample. The sample is known to be sandstone, limestone or granite.
The sample is 0.045 m thick. A probe sends a pulse through the sample. The pulse reflects from the opposite face and returns to the probe after 20 μs20\ \mu\text{s}20 μs.
The wave speed can be calculated using
v=2dtv=\dfrac{2d}{t}v=t2d
where
d d\,d is the thickness of the sample
t t\,t is the time taken for the echo to return.
The table gives measurements obtained from reference samples.
| Stone | Speed of ultrasound in m/s\text{m/s}m/s |
|---|---|
| sandstone | 3000 |
| limestone | 4500 |
| granite | 6000 |
The technician can use either of two probes to examine boundaries between thin layers in another stone sample.
| Probe | Wavelength in the stone in mm\text{mm}mm |
|---|---|
| R | 3.0 |
| S | 0.75 |
Give the echo time in seconds.
Calculate the speed of the ultrasound in the sample and use the reference measurements to identify the stone.
State which probe, R or S, should be used to distinguish boundaries that are close together.
Give a reason for your answer.
112 exam-style questions on Edexcel GCSE Physics 4.3 Sound, ultrasound and infrasound, covering 4.3.1 Sound waves, vibrations and the human ear, 4.3.2 Ultrasound and infrasound, 4.3.3 Uses of ultrasound and infrasound, and 4.3.4 Sound waves passing between media. Each one has a worked solution and a mark scheme showing where the marks go.