AQA · GCSE · Biology

    Course version: 8461 · Version from Sept 2018

    (HT only) Students should understand and use inverse proportion – the…

    Full specification wording

    Rate of photosynthesis: (HT only) Students should understand and use inverse proportion – the inverse square law and light intensity in the context of photosynthesis.

    Light intensity decreases with distance from a lamp because the light spreads out over a larger area.

    Checked against the AQA 8461 specification (opens in a new tab) on .

    • Point 5 of 9
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Light intensity decreases with distance from a lamp because the light spreads out over a larger area. The inverse square law states that light intensity is inversely proportional to the square of the distance: intensity ∝ 1 ÷ distance². If the distance doubles, the intensity falls to one quarter of its previous value; if the distance trebles, the intensity falls to one ninth. In photosynthesis, this means that moving a lamp twice as far away can reduce the rate if light is the limiting factor, but only while light remains limiting. To use the law, square the distance, then take the reciprocal. For example, at 0.50 m the intensity is proportional to 1 ÷ 0.50² = 4 arbitrary units, and at 1.00 m it is proportional to 1 ÷ 1.00² = 1 arbitrary unit. The law predicts the shape of a rate against distance graph, which rises steeply as distance decreases.

    What earns the marks

    • State that light intensity is inversely proportional to the square of the distance from a point source.
    • Use the relationship intensity ∝ 1 ÷ distance² to compare intensities at two distances.
    • Calculate the effect of doubling or trebling the distance on light intensity.
    • Explain that the inverse square law applies while light intensity is the limiting factor.
    • Interpret a graph of rate against distance in terms of the inverse square relationship.
    • Recognise that the law assumes a point source and no other limiting factors.

    Common mistakes and how to put them right

    • Treating intensity as inversely proportional to distance rather than to distance squared.

      Put it right by squaring the distance before taking the reciprocal.

    • Forgetting to square the distance when comparing two positions.

      Put it right by writing distance² in the calculation.

    • Assuming the rate always follows the inverse square law.

      Put it right by noting that other factors limit the rate at close distances or high intensities.

    Exam tips

    • Show the squared distance in your working before taking the reciprocal.
    • Use ratio reasoning: doubling distance gives one quarter of the intensity.
    • Link any calculated change in intensity to the rate only when light is limiting.

    Try the lesson

    The start of the Rate of photosynthesis: (HT only) Students should understand and use inverse proportion – the inverse square law and light intensity in the context of photosynthesis. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A lamp is moved so that its distance from a pondweed is doubled. According to the inverse square law, what happens to the light intensity at the pondweed?

    Listen

    Light intensity decreases with distance from a lamp because the light spreads out over a larger area. The inverse square law states that light intensity is inversely proportional to the square of the distance: intensity ∝ 1 ÷ distance². If the distance doubles, the intensity falls to one quarter of its previous value; if the distance trebles, the intensity falls to one ninth. In photosynthesis, this means that moving a lamp twice as far away can reduce the rate if light is the limiting factor, but only while light remains limiting. To use the law, square the distance, then take the reciprocal. For example, at 0.50 m the intensity is proportional to 1 ÷ 0.50² = 4 arbitrary units, and at 1.00 m it is proportional to 1 ÷ 1.00² = 1 arbitrary unit. The law predicts the shape of a rate against distance graph, which rises steeply as distance decreases. A strong answer earns marks for these points. State that light intensity is inversely proportional to the square of the distance from a point source. Use the relationship intensity ∝ 1 ÷ distance² to compare intensities at two distances. Calculate the effect of doubling or trebling the distance on light intensity. Explain that the inverse square law applies while light intensity is the limiting factor. A common slip: Treating intensity as inversely proportional to distance rather than to distance squared. Instead, squaring the distance before taking the reciprocal. Exam tip: Show the squared distance in your working before taking the reciprocal.

    See it

    A top-down flowchart with nine boxes connected by downward arrows. The first box says distance from lamp increases, leading to light spreads over a larger area, then light intensity decreases. This leads to the statement that intensity is inversely proportional to distance squared, then to square the distance, then take the reciprocal, then compare intensities at two distances. That leads to linking the change in intensity to the rate of photosynthesis, and finally to the note that this applies only while light is the limiting factor.
    It shows how increasing distance from a lamp reduces light intensity by the inverse square law and how this links to the rate of photosynthesis while light is limiting.

    Key words

    Say what each word means before you turn the card.

    inverse square law

    Try answering aloud or in your head before turning the card.

    light intensity

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    rate against distance graph

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    See every definition
    inverse square law
    Light intensity is inversely proportional to the square of the distance, so intensity ∝ 1 ÷ distance².
    light intensity
    A quantity that decreases with distance from a lamp because the light spreads out over a larger area.
    rate against distance graph
    A graph whose shape the law predicts, rising steeply as distance decreases.

    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 with nine boxes connected by downward arrows. The first box says distance from lamp increases, leading to light spreads over a larger area, then light intensity decreases. This leads to the statement that intensity is inversely proportional to distance squared, then to square the distance, then take the reciprocal, then compare intensities at two distances. That leads to linking the change in intensity to the rate of photosynthesis, and finally to the note that this applies only while light is the limiting factor.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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