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Waves and Particle Nature of Light

Displacement-distance and displacement-time graphs

  • 1 thing to know
  • Papers 2 and 3

Overview

Recall questions on what each axis shows, telling the two graphs apart, how a longitudinal wave is drawn, and identifying nodes and antinodes.

What you need to know

Answer each one before you reveal it.

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Numbers

Where do the compressions of a sound wave lie on a displacement graph, and how do you tell a displacement-distance graph from a displacement-time one?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What quantity is plotted on the horizontal axis of a displacement–distance graph, and what can you read from the graph?

    Model answer

    Distance along the direction of travel is plotted, and the graph gives the wavelength and the amplitude at that instant.

  2. 2

    What can be read from a displacement–time graph for a single point on a wave?

    Model answer

    It gives the period of the oscillation and the amplitude, but not the wavelength.

  3. 3

    A sine curve is shown with no axis labels. How would you tell whether it is a displacement–distance or a displacement–time graph?

    Model answer

    You must check the horizontal axis: a length scale means the repeat distance is a wavelength, while a time scale means the repeat interval is a period.

  4. 4

    How is a longitudinal wave represented on a displacement graph, given that the oscillation is along the direction of travel?

    Model answer

    The displacement of each molecule along the direction of travel is plotted as if it were a transverse displacement, with one sign for displacement forwards and the other for displacement backwards.

  5. 5

    On such a graph for a sound wave, where do the compressions lie?

    Model answer

    Compressions lie where the curve crosses the zero line with a negative gradient, because molecules on either side are displaced towards that point.

  6. 6

    How is a node identified on a displacement graph of a stationary wave?

    Model answer

    A node is a position where the displacement is permanently zero, so every trace of the wave crosses the axis at that same point.

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