Reflection, transmission and pulse-echo techniquesEdexcel International A Level Physics: Subtopic test

1. 1
   A hospital sonographer scans a patient using an ultrasound probe that emits very short pulses of ultrasound. A layer of gel is spread between the probe and the patient's skin. The same probe detects the pulses that are reflected back from inside the body.

   (a)

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

   [1 mark]

   • AIt is entirely absorbed at the boundary
   • BIt is entirely reflected back towards the probe
   • CPart is reflected and part is transmitted
   • DIt is entirely transmitted, with a change in frequency

   (b)

   What is the purpose of the gel between the probe and the skin?

   [1 mark]

   • ATo cool the probe
   • BTo increase the frequency of the pulses
   • CTo make the pulses shorter
   • DTo remove air, so that little ultrasound is reflected at the skin surface

   (c)

   Explain why the probe emits short pulses rather than a continuous beam of ultrasound.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A ship uses an echo sounder to measure the depth of the sea. It sends a short pulse of ultrasound downwards and detects the echo from the sea bed. The speed of ultrasound in seawater is 1.5 × 10³ m s⁻¹. A pulse sent by the ship returns from the sea bed 0.080 s after it was emitted.

   (a)

   Which expression gives the depth d of the sea bed, where v is the speed of ultrasound and t is the time between emission and detection of the echo?

   [1 mark]

   • Ad = vt
   • Bd = vt / 2
   • Cd = 2vt
   • Dd = v / 2t

   (b)

   The echo from the sea bed is much weaker than the pulse that was emitted. Which is a reason for this?

   [1 mark]

   • ASome of the energy is transmitted into the sea bed and some is absorbed in the water
   • BThe frequency of the pulse decreases when it is reflected
   • CThe speed of the pulse decreases on the way back
   • DThe wavelength of the pulse increases at the boundary

   (c)

   Calculate the depth of the sea bed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A medical ultrasound probe operates at a frequency of 2.5 MHz. The speed of ultrasound in soft tissue is 1540 m s⁻¹. A doctor wants to image a small structure in soft tissue that is 0.30 mm wide.

   (a)

   Calculate the wavelength of the ultrasound in the tissue.

   [3 marks]

   (b)

   Deduce whether this probe can resolve the 0.30 mm structure, and suggest, with a calculation, a change that would allow it to be seen.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An engineer inspects a steel plate using a pulse-echo ultrasonic tester. The speed of ultrasound in steel is 5900 m s⁻¹. The tester sends pulses into the plate, and echoes return from the back surface and from any internal defects.

   (a)

   The pulses last 4.0 μs. The back surface of a thin layer in the plate is 5.0 mm behind the front surface of the layer. Calculate the time delay between the two echoes, deduce whether the tester can show them as separate echoes, and suggest a change that would allow it.

   [6 marks]

   (b)

   The engineer wants to detect a void 0.10 mm wide inside the plate using ultrasound of frequency 5.0 MHz. Deduce whether this frequency is suitable and calculate the minimum frequency that would be needed.

   [6 marks]

   Total for question 4: 12 marks

End of questions