AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Qualitative treatment of the variation of the width of the central…

    Full specification wording

    Diffraction: Qualitative treatment of the variation of the width of the central diffraction maximum with wavelength and slit width.

    The width of the central diffraction maximum depends on both the wavelength of the light and the width of the slit.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 2 of 5
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The width of the central diffraction maximum depends on both the wavelength of the light and the width of the slit. For a given slit width, increasing the wavelength makes the central maximum wider, so red light produces a broader central maximum than blue light. For a given wavelength, increasing the slit width makes the central maximum narrower, so a wider slit gives a tighter pattern. This is a qualitative relationship: the central maximum is widest when the wavelength is large compared with the slit width, and narrowest when the slit is wide compared with the wavelength. A practical way to see this is to shine a laser through a single slit and gradually narrow the slit; the central maximum spreads out as the slit becomes narrower.

    What earns the marks

    • For a fixed slit width, the central maximum becomes wider as the wavelength increases.
    • For a fixed wavelength, the central maximum becomes narrower as the slit width increases.
    • The central maximum is widest when the slit width is small compared with the wavelength.
    • The central maximum is narrowest when the slit width is large compared with the wavelength.
    • The relationship is qualitative: no calculation is required, but the direction of change must be stated correctly.

    Common mistakes and how to put them right

    • Stating that increasing the slit width makes the central maximum wider.

      Put it right: Increasing the slit width makes the central maximum narrower.

    • Stating that increasing the wavelength makes the central maximum narrower.

      Put it right: Increasing the wavelength makes the central maximum wider.

    • Confusing the width of the central maximum with its brightness.

      Put it right: Brightness and width are separate features, and the question asks about width.

    Exam tips

    • Use comparative language such as 'wider', 'narrower', 'increases' and 'decreases' to make the direction of change clear.
    • Link the effect to the ratio of wavelength to slit width, for example by saying that a smaller slit width relative to the wavelength gives a wider central maximum.
    • When comparing red and blue light through the same slit, state that red light gives a wider central maximum than blue light because red has a longer wavelength.

    Try the lesson

    The start of the Diffraction: Qualitative treatment of the variation of the width of the central diffraction maximum with wavelength and slit width. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A single slit is made narrower while the wavelength of the light is kept the same. What happens to the width of the central diffraction maximum?

    Listen

    The width of the central diffraction maximum depends on both the wavelength of the light and the width of the slit. For a given slit width, increasing the wavelength makes the central maximum wider, so red light produces a broader central maximum than blue light. For a given wavelength, increasing the slit width makes the central maximum narrower, so a wider slit gives a tighter pattern. This is a qualitative relationship: the central maximum is widest when the wavelength is large compared with the slit width, and narrowest when the slit is wide compared with the wavelength. A practical way to see this is to shine a laser through a single slit and gradually narrow the slit; the central maximum spreads out as the slit becomes narrower. A strong answer earns marks for these points. For a fixed slit width, the central maximum becomes wider as the wavelength increases. For a fixed wavelength, the central maximum becomes narrower as the slit width increases. The central maximum is widest when the slit width is small compared with the wavelength. The central maximum is narrowest when the slit width is large compared with the wavelength. A common slip: Stating that increasing the slit width makes the central maximum wider. Instead, increasing the slit width makes the central maximum narrower. Exam tip: Use comparative language such as 'wider', 'narrower', 'increases' and 'decreases' to make the direction of change clear.

    See it

    A top-down flowchart. The top node, central diffraction maximum width, branches into two nodes: depends on wavelength and depends on slit width. From depends on wavelength, an arrow leads to increase wavelength at fixed slit width, which leads to central maximum becomes wider. From depends on slit width, an arrow leads to increase slit width at fixed wavelength, which leads to central maximum becomes narrower. The wider node then leads to widest when slit width is small compared with wavelength, and the narrower node leads to narrowest when slit width is large compared with wavelength.
    It shows how the width of the central diffraction maximum changes with wavelength and slit width, and the extremes of the wavelength-to-slit-width ratio.

    Key words

    Say what each word means before you turn the card.

    Wavelength dependence of diffraction

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    Slit width and diffraction spreading

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    Qualitative reasoning in wave optics

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    Experimental control of diffraction variables

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    See every definition
    Wavelength dependence of diffraction
    For a given slit width, increasing the wavelength makes the central maximum wider, so red light produces a broader central maximum than blue light.
    Slit width and diffraction spreading
    For a given wavelength, increasing the slit width makes the central maximum narrower, so a wider slit gives a tighter pattern.
    Qualitative reasoning in wave optics
    The relationship is qualitative: no calculation is required, but the direction of change must be stated correctly.
    Experimental control of diffraction variables
    Shining a laser through a single slit and gradually narrowing the slit shows the central maximum spreading out as the slit becomes narrower.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The top node, central diffraction maximum width, branches into two nodes: depends on wavelength and depends on slit width. From depends on wavelength, an arrow leads to increase wavelength at fixed slit width, which leads to central maximum becomes wider. From depends on slit width, an arrow leads to increase slit width at fixed wavelength, which leads to central maximum becomes narrower. The wider node then leads to widest when slit width is small compared with wavelength, and the narrower node leads to narrowest when slit width is large compared with wavelength.

    • 1Box 1

    • 2Box 2

    • 3Box 3

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