Waves at interfaces and pulse-echoEdexcel A-Level Physics: Subtopic test

1. 1
   A sonographer is carrying out an ultrasound scan of a patient. She applies a layer of gel to the skin before pressing the transducer against it. The transducer emits ultrasound of frequency 3.5 MHz, which travels through soft tissue at 1540 m s⁻¹.

   (a)

   Why does the sonographer apply gel between the transducer and the skin?

   [1 mark]

   • ATo cool the transducer
   • BTo increase the wavelength of the ultrasound
   • CTo remove the air gap, which would reflect almost all the ultrasound at the skin
   • DTo absorb stray ultrasound waves

   (b)

   What happens to an ultrasound pulse when it meets the boundary between two different tissues?

   [1 mark]

   • APart of it is reflected and part is transmitted
   • BAll of it is reflected
   • CAll of it is transmitted unchanged
   • DIts frequency changes to that of the second tissue

   (c)

   Calculate the wavelength of the ultrasound in soft tissue.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a pulse-echo scan, a transducer sends a short pulse of ultrasound into soft tissue, where the speed of ultrasound is 1540 m s⁻¹. An echo from the boundary of an organ is detected 78 μs after the pulse was transmitted.

   (a)

   How deep is the boundary of the organ below the transducer?

   [1 mark]

   • A12 cm
   • B6.0 cm
   • C3.0 cm
   • D0.60 cm

   (b)

   The echo is much weaker than the pulse that was sent. Which statement gives the best explanation?

   [1 mark]

   • AThe frequency of the ultrasound decreases on reflection
   • BAll of the pulse is reflected, so the echo carries the same energy
   • CThe speed of ultrasound is lower on the return journey
   • DOnly part of the pulse is reflected at the boundary and the pulse is also absorbed in the tissue

   (c)

   The transducer sends pulses at regular intervals. Explain why the interval between pulses must be longer than the time for the echo from the deepest boundary to return.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A pulse-echo system uses ultrasound of frequency 5.0 MHz. Each pulse lasts 0.40 μs. The speed of ultrasound in the tissue being scanned is 1540 m s⁻¹.

   (a)

   Calculate the wavelength of the ultrasound and the length of one pulse in the tissue.

   [3 marks]

   (b)

   Two boundaries in the tissue are 0.20 mm apart along the direction of the beam. Explain, with calculation, whether the system produces separate echoes from them, and state how the system could be changed to distinguish them.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An ultrasound scanner uses ultrasound of frequency 8.0 MHz in short pulses, each lasting 0.25 μs, to examine tissue in which the speed of ultrasound is 1540 m s⁻¹. The operator wants to use the scanner to look at fine structures beneath the skin.

   (a)

   Explain how a pulse-echo system uses ultrasound to build up an image of boundaries between different tissues, and why gel is applied to the skin.

   [6 marks]

   (b)

   Determine the wavelength of the ultrasound and the length of one pulse in the tissue. Evaluate whether the scanner can (i) detect a calcium deposit of width 0.10 mm and (ii) distinguish two boundaries 0.50 mm apart along the beam. Suggest one change to improve the resolution and state one disadvantage of it.

   [6 marks]

   Total for question 4: 12 marks

End of questions