Core practical: light intensity and photosynthesis

Core practical 6.5 investigates how light intensity affects the rate of photosynthesis, using algal balls (or pondweed) and a lamp at different distances.

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Algal balls and indicator

Algal balls are tiny algae trapped in jelly beads. They are placed in hydrogencarbonate indicator, which changes colour with the carbon dioxide level:
yellow = high carbon dioxide; orange-red = normal (air); purple = low carbon dioxide.
Photosynthesis uses up carbon dioxide, so the faster the rate, the faster the indicator turns purple.

The method

1. Put the same number of algal balls in the same volume of indicator in several tubes.
2. Place the tubes at different distances from a lamp, for example 10, 20, 30 and 40 cm.
3. Leave them for the same time, then compare the colours with a set of standard colours (or measure the pH or use a colorimeter).
Control variables: temperature (a beaker of water between lamp and tube can absorb heat), number of algal balls, volume of indicator, time.
A tube covered in foil is a useful control.

Inverse square law (Higher)

Light intensity is inversely proportional to the square of the distance from the lamp:
light intensity ∝ 1 ÷ d2
Doubling the distance gives one quarter of the intensity. Tripling it gives one ninth.
Worked example

A lamp is moved from 10 cm to 30 cm from a tube. By what factor does the light intensity change?

  1. The distance is multiplied by 30 ÷ 10 = 3.
  2. Intensity ∝ 1 ÷ d2, so it is divided by 32 = 9.

Answer: The light intensity becomes one ninth (it is divided by 9).

Key idea

Algal balls in hydrogencarbonate indicator turn purple faster when photosynthesis is faster. Move the lamp to change light intensity and keep temperature, number of balls and time the same. Light intensity ∝ 1 ÷ d2 (Higher).

Key words
Photosynthesis
The process in which plants use light energy to make glucose and oxygen from carbon dioxide and water.
Hydrogencarbonate indicator
An indicator that turns yellow when carbon dioxide concentration rises and purple when it falls.

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Quick check

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Extra practice

2 more questions

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  1. Hydrogencarbonate indicator around algal balls stays orange-red. What does this show?

    1. Carbon dioxide has increased
    2. Carbon dioxide has decreased
    3. The algae are dead
    4. The rates of photosynthesis and respiration are equal
    Show the answer

    Answer: The rates of photosynthesis and respiration are equal

    No net change in carbon dioxide.

  2. A tube of algal balls is placed in bright light. The indicator turns purple. Why?

    1. The light reacts with the indicator
    2. Photosynthesis is faster than respiration, so carbon dioxide falls
    3. Respiration is faster
    4. The algae died
    Show the answer

    Answer: Photosynthesis is faster than respiration, so carbon dioxide falls

    Carbon dioxide is used up.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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