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How to Measure the Effect of Light on Photosynthesis

Photosynthesis is a process in plants that converts carbon dioxide into complex organic materials using sunlight. But changing the amount of light available to the plant has a reaction in the photosynthesis of that organism. It may be that you have been assigned the task of finding out exactly how much of an effect this is for a science experiment at school. Or perhaps your school days are long gone but you are still brimming full of scientific curiosity. Whatever the reason, testing the effects of light on the process of photosynthesis is not as difficult as it sounds.

Things You'll Need

8 test tubes
4 test-tube racks
2 straws
paper towel
Knife
Bromthymol blue indicator solution
Tape measure
Ruler
Floodlight
Lab apron, gloves and safety goggles.
Dropper
4 sprigs of evergreen plant
Distilled water
Pen
Paper
Stopwatch

Instructions

- 1
  Put on your lab apron, safety goggles and gloves.
- 2
  Insert two test tubes into each of the four test tube racks.
- 3
  Fill each test tube with water, leaving about 2 inches inch at the top of the test tube unfilled
- 4
  Add a drop of Bromthymol blue indicator solution (BTB) and then use a straw to stir the test tube. Keep adding a drop of BTB -- counting each drop -- and then stirring the mixture until it turns a pale blue color.
- 5
  Repeat Step 4 for each of the remaining seven test tubes.
- 6
  Blow softly into the one of the test tube with the unused straw until it turns a pale yellow color, but do not, under any circumstances, inhale any of the test tube solution. Blowing into the test tube adds carbon dioxide to the solution.
- 7
  Repeat the previous step for all of the test tubes, making sure to blow with the same end of the straw.
- 8
  Lay out four evergreen plant sprigs on a paper towel. Slice off 0.2 inches from the base of the sprig.
- 9
  Align the plants to see if they are roughly the same size; if not then cut more from the longer sprigs until all plants looks even.
- 10
  Drop one sprig into one of the test tubes on each rack. Make sure that the part of the sprig that you previously cut is at the bottom of the test tube.
- 11
  Position one rack 25 inches away from the floodlight. The next rack should be 20 inches away, then 15 inches away, then 10 inches away. Place the last remaining test tube rack 5 inches away from the flood light.
- 12
  Turn on the floodlight.
- 13
  Start a stopwatch.
- 14
  Write down the time on the stopwatch every time one of the test tubes changes color. Compare the results of the test tube placed close to the light to those further away and also how long it took the test tubes with the springs in to change color compared to the sprig-free tubes.

Nature