A student is investigating the rate of photosynthesis in a suspension of the green alga Chlorella immobilised in hydrogencarbonate indicator solution.
To manipulate light intensity, they vary the distance of a high-output incandescent spotlight from the glass tube containing the algae. However, they observe that when the spotlight is positioned closest to the tube, the temperature of the indicator solution rises, which could confound the photosynthesis rate measurements.
Identify one modification the student could make to their experimental design to prevent this temperature increase from compromising the validity of the experiment.
