This experiment will need careful setting up to give you valid results.
First you will need to source some pond weed, Elodea or Cabomba will work. You will also need a large beaker or ice cream tub and a clear measuring cylinder - a glass one would be best. Finally you will need a lamp, a ruler and a stopclock.
The pond weed should be placed into the measuring cylinder and then this should be filled with water in the ice cream tub and then inverted so that the measuring cylinder is full of water. Set the lamp up at a known distance from the pond weed and start the stop clock.
It is up to you how long you leave the equipment set up like this, however the longer you can leave it the more gas will be produced. You will need to record the volume of gas produced when you return to the experiment.
Replace the water in the measuring cylinder set up, move the lamp to a new distance from the pond weed and then leave the equipment for the same time as before.
Repeat again for at least one more distance between lamp and pond weed, however if you can do more then do.
If you are leaving the equipment set up for a significant length of time unattended you will need to make sure that the measuring cylinder is supported - especially if it is made of glass. You should also ensure that there is nothing resting on or near the lamp as depending on the type of bulb used the lamp could become hot after extended periods of operation.