5.3.1 Intensity, reflection and pulse-echo techniques Quiz: Pearson Edexcel Physics, Unit 5

The 20 questions

1. What is the SI unit for the intensity of a wave?

   • J m⁻²
   • W m⁻¹
   • W m⁻²
   • N m⁻²

2. Which equation defines wave intensity I in terms of power P and area A?

   • I = A / P
   • I = P / A²
   • I = P / A
   • I = P × A

3. How does wave intensity I vary with distance r from a point source?

   • Proportional to r
   • Proportional to r²
   • Proportional to 1/r²
   • Proportional to 1/r

4. Which property mismatch determines the fraction of ultrasound reflected at a boundary?

   • Acoustic impedance
   • Refractive index
   • Wave amplitude
   • Particle velocity

5. In pulse-echo ranging, how is the distance to a reflector found from the echo time t and wave speed v?

   • d = t/(2v)
   • d = v t/2
   • d = v/t
   • d = v t

6. What type of pulse duration improves spatial resolution in pulse-echo imaging?

   • Continuous pulse
   • Infinite pulse duration
   • Long pulse duration
   • Short pulse duration

7. What is a main advantage of using higher frequency ultrasound in medical imaging?

   • Better resolution
   • Lower attenuation
   • Greater wave speed
   • Deeper penetration

8. A source transfers 2.0 W of power through an area of 0.50 m^2 normal to the direction of travel. What is the intensity?

   • 2.5 W/m^2
   • 1.0 W/m^2
   • 0.25 W/m^2
   • 4.0 W/m^2

9. The intensity at 2.0 m from a point source is 9.0 W/m^2. What is the intensity at 6.0 m?

   • 1.0 W/m^2
   • 3.0 W/m^2
   • 27 W/m^2
   • 0.33 W/m^2

10. An ultrasound pulse in tissue, with speed 1500 m/s, returns an echo after 80 microseconds. How far away is the reflector?

    • 3.0 cm
    • 12 cm
    • 1.2 cm
    • 6.0 cm

11. In water, with sound speed 1500 m/s, an echo returns after 0.20 ms. What is the depth of the reflector?

    • 0.075 m
    • 0.30 m
    • 0.15 m
    • 0.60 m

12. Why is gel placed between an ultrasound probe and skin?

    • Increases wave speed
    • Absorbs reflected pulses
    • Increases beam intensity
    • Reduces impedance mismatch

13. A point source emits 40 W uniformly in all directions. What is the intensity at a distance of 2.0 m?

    • 0.80 W/m^2
    • 3.2 W/m^2
    • 10 W/m^2
    • 0.40 W/m^2

14. A pulse lasts 2.0 microseconds in tissue where the sound speed is 1500 m/s. What is the spatial length of the pulse?

    • 1.5 mm
    • 0.75 mm
    • 3.0 mm
    • 30 mm

15. A reflector lies 3.0 cm deep in tissue where the sound speed is 1500 m/s. How long after the pulse is emitted is its echo received?

    • 40 microseconds
    • 4 microseconds
    • 80 microseconds
    • 20 microseconds

16. How does using shorter ultrasound pulses affect image quality?

    • Improves resolution
    • Reduces resolution
    • Decreases frequency
    • Increases penetration

17. A 60 W isotropic point source is used. At what distance is the intensity 0.10 W/m^2?

    • About 2.2 m
    • About 48 m
    • About 6.9 m
    • About 3.5 m

18. Two reflectors in tissue give echoes at 20 microseconds and 35 microseconds. Take v = 1500 m/s. What is the separation of the reflectors?

    • 2.2 cm
    • 5.6 cm
    • 0.56 cm
    • 1.1 cm

19. Why are megahertz frequencies used instead of kilohertz in medical ultrasound?

    • Shorter wavelength
    • Greater penetration
    • Lower wave speed
    • Longer wavelength

20. Which process besides geometric spreading reduces ultrasound intensity as it travels through tissue?

    • Diffraction
    • Attenuation
    • Dispersion
    • Refraction

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