Lesson 5.3.1

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

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 5.3.1, Intensity, reflection and pulse-echo techniques: 20 multiple choice questions for the Pearson Edexcel Physics (9PH0), Unit 5: Waves and Particle Nature of Light, written with Revision Ninja.

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The 20 questions

  1. What is the intensity of a wave?

    • Energy per unit time per unit length, measured in J/m
    • Amplitude squared per unit time, measured in m^2/s
    • Power per unit area perpendicular to the direction of energy flow, measured in W/m^2
    • Force per unit area, measured in N/m^2

  2. Which equation defines the intensity I of a wave?

    • I = A/P
    • I = P/A^2
    • I = P/A, where P is the power and A the area normal to the wave
    • I = P A

  3. How does the intensity from a point source vary with distance r from the source, in the absence of absorption?

    • It is independent of r
    • It is proportional to r^2
    • It is proportional to 1/r^2
    • It is proportional to 1/r

  4. What determines how much of a wave is reflected at an interface between two media?

    • The difference in the relevant property of the two media, such as acoustic impedance or refractive index
    • The frequency of the wave alone
    • The amplitude of the wave alone
    • The colour of the medium

  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 are the two key conditions for a pulse-echo system to resolve two closely spaced reflectors?

    • A long pulse and a short wavelength
    • A short pulse and a long wavelength
    • A long pulse and a long wavelength
    • A short pulse and a short wavelength, relative to the separation of the reflectors

  7. What is the effect of using a higher ultrasound frequency in medical imaging?

    • Shorter wavelength and better resolution, but greater attenuation in tissue
    • Shorter wavelength but poorer resolution
    • No change in resolution, but deeper penetration into tissue
    • Longer wavelength and better resolution, with less attenuation

  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 a layer of gel placed between an ultrasound probe and the skin?

    • It increases the speed of sound in tissue to match the probe
    • It absorbs ultrasound to protect the patient
    • It lowers the frequency of the ultrasound
    • It removes air gaps, which would reflect almost all of the ultrasound because of their large 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. A student claims a pulse-echo system gives finer detail when the pulse is longer. Which evaluation is correct?

    • Incorrect, because shorter pulses give finer detail, since the pulse length limits the smallest resolvable separation
    • Correct, because longer pulses carry more energy to each reflector
    • Incorrect, because the pulse width has no effect on the information obtained
    • Correct, because wider pulses resolve smaller reflectors

  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 do medical scanners use ultrasound frequencies of the order of megahertz rather than kilohertz?

    • Shorter wavelengths give finer detail, while attenuation is still acceptable over depths of a few centimetres
    • Kilohertz waves are completely absorbed by bone
    • Megahertz waves do not reflect from tissue boundaries
    • Higher-frequency waves travel faster in tissue

  20. Why does the intensity of an ultrasound beam fall with depth in tissue more than inverse-square spreading alone would predict?

    • Intensity of ultrasound is independent of depth in tissue
    • Tissue absorbs and scatters energy, so the beam loses intensity in addition to spreading
    • Reflections at the surface raise the intensity with depth
    • Ultrasound speeds up as it travels, so its intensity falls

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