Cut an incision into the stem on the Elodea leaf. Fill the test tube with room temperature water and place the Elodea leaf in the test tube so that it is fully submerged and the stem is pointing toward the top of the tube. Weigh the leaf down if necessary and place the test tube in a test tube rack.
Attach the 40-watt bulb to the light source and place 20 inches from the test tube rack. The light source should be pointing directly at the Elodea leaf in the test tube.
Switch on the light source and simultaneously set the timer. Count the oxygen bubbles produced from the incision in the stem for 10 seconds, and then repeat the process for 30 seconds and one minute, recording all observations as you go.
Repeat steps 1, 2 and 3 with the 60- and 100-watt bulbs and record how many bubbles were produced in each time period. Results should be plotted in a graph, which will illustrate the effect of light intensity on the rate of photosynthesis.
Using the 40-watt bulb again, cover the light source with the green cellophane and count how many bubbles are produced after 30 seconds. Then repeat the experiment again for 30 seconds, but this time cover the light source with red cellophane. Observe the differences between the results with the colored light filters and the results without. Engage the students by asking them to discuss their findings and relate them to the principle of photosynthesis.