Light intensity, distance and the inverse square lawSpec 6.6
In short
The inverse square law states that light intensity is proportional to 1 ÷ distance². When light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity, so it follows the same law. Doubling the distance from the lamp makes the intensity, and the rate, one quarter. The rule only works while light is limiting.
When light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity. If the light intensity doubles, the rate doubles.
The light intensity falls as the distance from the light source increases. So the rate of photosynthesis is inversely proportional to the distance from the light source. The exact relationship is the inverse square law: light intensity is proportional to 1 ÷ distance², so the rate is too.
- Double the distance: the intensity becomes 1 ÷ 2² = one quarter.
- Treble the distance: the intensity becomes 1 ÷ 3² = one ninth.
- Halve the distance: the intensity becomes 1 ÷ (½)² = four times as much.
Using the inverse square law
A pondweed 10 cm from a lamp releases 36 bubbles per minute. Light is the limiting factor. Predict the number of bubbles per minute when the lamp is moved to 30 cm.
- The distance has increased by a factor of 30 ÷ 10 = 3.
- Light intensity is proportional to 1 ÷ distance², so the intensity falls by a factor of 3² = 9.
- The rate is directly proportional to light intensity, so the rate also falls by a factor of 9.
- New rate = 36 ÷ 9 = 4 bubbles per minute.
Answer: 4 bubbles per minute
The rule only works while light is the limiting factor. Very close to the lamp, another factor such as carbon dioxide concentration or temperature takes over, and the rate stops increasing.
Quick check
Why are photosynthetic organisms called the main producers of biomass?
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They make their own food (glucose) by photosynthesis, which builds biomass that other organisms feed on.
Why is photosynthesis described as endothermic?
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It takes in energy from the surroundings, in the form of light.
Name the three limiting factors of photosynthesis.
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Temperature, light intensity and carbon dioxide concentration.
In the light intensity core practical, how is the light intensity changed?
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By changing the distance between the lamp and the pondweed.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026