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Wave Frequency

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Wave Frequency

Imagine making transverse waves in a rope, like the person in the sketch above. You tie one end of the rope to a tree or other fixed point, and then you shake the other end of the rope up and down with your hand. You can move the rope up and down slowly or quickly. How quickly you move the rope determines the frequency of the waves.

What Is Wave Frequency?

The number of waves that pass a fixed point in a given amount of time is wave frequency . Wave frequency can be measured by counting the number of crests (high points) of waves that pass the fixed point in 1 second or some other time period. The higher the number is, the greater the frequency of the waves. The SI unit for wave frequency is the hertz (Hz) , where 1 hertz equals 1 wave passing a fixed point in 1 second. The Figure below shows high-frequency and low-frequency transverse waves. You can simulate transverse waves with different frequencies at these URLs: http://phet.colorado.edu/en/simulation/wave-on-a-string and http://zonalandeducation.com/mstm/physics/waves/partsOfAWave/waveParts.htm

Q: The wavelength of a wave is the distance between corresponding points on adjacent waves. For example, it is the distance between two adjacent crests in the transverse waves in the diagram. Infer how wave frequency is related to wavelength.

A: Waves with a higher frequency have crests that are closer together, so higher frequency waves have shorter wavelengths.

Wave Frequency and Energy

The frequency of a wave is the same as the frequency of the vibrations that caused the wave. For example, to generate a higher-frequency wave in a rope, you must move the rope up and down more quickly. This takes more energy, so a higher-frequency wave has more energy than a lower-frequency wave with the same amplitude. You can see examples of different frequencies in the Figure below (Amplitude is the distance that particles of the medium move when the energy of a wave passes through them.)

Summary

  • Wave frequency is the number of waves that pass a fixed point in a given amount of time.
  • The SI unit for wave frequency is the hertz (Hz), where 1 hertz equals 1 wave passing a fixed point in 1 second.
  • A higher-frequency wave has more energy than a lower-frequency wave with the same amplitude.

Vocabulary

  • hertz (Hz) : SI unit of wave frequency, where 1 hertz equals 1 wave passing a fixed point per second.
  • wave frequency : Number of waves that pass a fixed point in a given amount of time.

Practice

At the following URL, use the wave simulator to make waves with various amounts of energy. Measure the frequency and wavelength of the waves, and check your measurements. Then answer the question at the bottom of the Web page. http://amazing-space.stsci.edu/resources/explorations/light/makewaves-frames.html

Review

  1. What is wave frequency?
  2. What is the SI unit for wave frequency?
  3. Assume that 10 waves pass a fixed point in 5 seconds. What is the frequency of the waves in hertz?
  4. Relate wave frequency to the energy of waves.

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