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A LevelPhysicsCIERevision Notes7. Waves7.1 Progressive WavesCathode-Ray Oscilloscope

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First teaching 2023

First exams 2025

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Cathode-Ray Oscilloscope (CIE A Level Physics)

Revision Note

Katie M

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Katie M

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Cathode-ray oscilloscope

  • A cathode-ray oscilloscope is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits
  • An a.c. current on an oscilloscope is represented as a transverse wave
    • The oscilloscope can be used to determine its frequency and amplitude
  • A d.c voltage on an oscilloscope is represented as a horizontal line at the relevant voltage
  • The x-axis is the time and the y-axis is the voltage (or y-gain)

 

Cathode-ray oscilloscope display

CRO diagram, downloadable AS & A Level Physics revision notes

Diagram of cathode-ray oscilloscope display showing wavelength and time-base setting

 

  • The period of the wave can be determined from the time-base
    • Time-base is how many seconds each division represents measured commonly in s div-1 or s cm-1
  • Count the number of wavelengths shown on the screen 
  • Divide the total time by the number of wavelengths to obtain the time period T 
  • Calculate the frequency of the wave using f space equals space 1 over T

 

Worked example

A cathode-ray oscilloscope (c.r.o.) is used to display the trace from a sound wave. The time-base is set at 7 µs mm-1.WE - CRO question image, downloadable AS & A Level Physics revision notesWhat is the frequency of the sound wave?

A    2.4 Hz          B     24 Hz          C     2.4 kHz          D     24 kHz

Answer: C

Step 1: Determine the period of the wave:

  • One full wave covers three horizontal squares, which is a distance of 6 cm
  • There are 60 mm in 6 cm
    • Each mm represents a time of 7 μs
    • Time period, T = 60 × (7 × 10−6) = 4.2 × 10−4 s

Step 2: Use the frequency equation to determine frequency:

  • Recall the frequency equation

f space equals space 1 over T space equals space fraction numerator 1 over denominator 4.2 space cross times space 10 to the power of negative 4 end exponent end fraction space equals space 2380 space Hz

  • Recall that 1000 Hz = 1 kHz and the answer must be given to 2 significant figures

f space equals space 2.4 space kHz

Examiner Tip

The time-base setting varies with units for seconds (commonly ms) and the unit length (commonly mm). Unit conversions are very important for the calculation of the time period and frequency

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    Katie M

    Author: Katie M

    Expertise: Physics

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.