Practical to investigate how light intensity effects photosynthesis

1)Fill boiling tube 45cm^2 sodium hydrocarbonate soloution, leave for a few mins then shake air bubbles out. 2)Cut 8cm pondweed and place carefullt in boiling tube. 3)Place 10cm away from light source count n.o. bubbles for 5 minutes. 5)repeat at further distanes at interval of 5cm. 4)repeat 5 times and calculate average mean.