Photosynthesis - AQA

Plants make their own food using photosynthesis. The food that plants produce is important, not only for the plants themselves, but for the other organisms that feed on the plants.

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Combined Science

Bioenergetics

Scientific calculations – Higher only

There is an inverse relationship between distance and light intensity – as the distance increases, light intensity decreases.

This is because as the distance away from a light source increases, photons of light become spread over a wider area.

An image showing the relationship between distance and light intensity.

The light energy at twice the distance away is spread over four times the area.

The light energy at three times the distance away is spread over nine times the area, and so on.

The light intensity is inversely proportional to the square of the distance – this is the inverse square law.

For each distance of the plant from the lamp, light intensity will be proportional to the inverse of $d^{2}$ , $d^{2}$ meaning distance squared.

Calculating $\frac{1}{d^{2}}$ :

For instance, for the lamp 10 cm away from the plant:

$\frac{1}{d^{2}} = \frac{1}{10^{2}} = \frac{1}{100} = 0.01$

If we refer back to the data the students collected from the experiment:

Distance	Rate
10 cm	120
15 cm	54
20 cm	30
25 cm	17
30 cm	13

Completing the results table:

Distance	$\frac{1}{d^{2}}$	Rate
10 cm	0.0100	120
15 cm	0.0044	54
20 cm	0.0025	30
25 cm	0.0016	17
30 cm	0.0011	13

If we plot a graph of the rate of reaction over $\frac{1}{d^{2}}$ :

A graph showing the number of bubbles produced per minute.

The graph is linear.

The relationship between light intensity – at these low light intensities – is linear.

Be careful – the x-axis is values of $\frac{1}{d^{2}}$ . It is not of light intensity.

$\frac{1}{d^{2}}$ is proportional to light intensity.

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Photosynthesis - AQA

Scientific calculations – Higher only

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