Required Practical: Light Intensity and the Rate of Photosynthesis

The rate of photosynthesis in a plant can be measured using a simple experiment.


1. Set up the apparatus above, placing the funnel upside down in the beaker

  • But do not put the measuring tube in the beaker until the pondweed has adjusted

2. Cut a piece of pondweed around 7 cm long and use forceps to place it in the beaker

3. Use a ruler to place a lamp 15 cm from the beaker containing pondweed

  • The lamp does not have to be placed like it is in the diagram, as long as it is directed at the pondweed in the beaker

4. Leave the apparatus for around 10 minutes to allow the pondweed to adjust

5. Place the boiling tube upside down in the beaker, over the upside-down funnel

  • We will use the boiling tube to count the number of bubbles released, which is oxygen gas

6. Count the number of bubbles in the boiling tube after one minute

7. Repeat the count five times, recording your results in a table, such as the one below:

DistanceNumber of bubbles (1)Number of bubbles (2)Number of bubbles (3)
15 cm
20 cm
25 cm
30 cm
35 cm

8. Repeat the experiment at different distances from the lamp

9. Plot the results on a graph to see a pattern or trend

You can use a gas syringe to collect the volume of oxygen produced by the pondweed instead of counting the bubbles produced. This will give you more accurate results. If you decide to take this approach, you can plot a graph:

  • X-axis = Distance from the lamp
  • Y-axis = Change in gas volume


Independent variable – The distance from the lamp (light intensity)

Dependent variable – The number of bubbles (oxygen) produced per minute

Control variables – Temperature and using the same piece of pondweed

Risk Control

Dry your hands before handling the lamp.