Re: Color of food coloring in water affect photosynthesis

If you can, it would be best to use two beakers, one inside the other. Put the 
Elodea in the smaller beaker and place it inside a large beaker that has the 
food coloring solution in it. That way there is no possibility the food 
coloring is having a toxic effect on the Elodea, it will just be changing the 
light color the Elodea receives. You may need to place an opaque cover on the 
Elodea beaker to assure all the light the Elodea gets is passing through the 
food coloring solution.

If your school has a spectrophotometer, it would be desirable to do a 
transmission graph for each food coloring solution. Measure the percent 
transmission every 5 or 10 nanometers between wavelengths of 400 and 700 
nanometers (equivalent to 0.4 to 0.7 micrometers). Graph the percent 
transmission on the vertical axis versus wavelength on the horizontal axis. 
The transmission graphs are useful because the human eye is not a good light 
sensor. A green food coloring solution will transmit mainly green wavelengths, 
but it may be transmiting other colors as well. The human eye is most senstive 
to green wavelengths and much less sensitive to red and blue wavelengths.

It is hard to predict your results because they will depend on the light 
transmission of the various food coloring solutions and the light source. For 
example, incandescent bulbs are very rich in red wavelengths compared to 
sunlight and fluorescent lamps.

You may find slightly lower photosynthesis with green or yellow compared to 
red or blue but that is far from guaranteed. Contrary to many books, leaves do 
not reflect all green light. Check a 
photosynthesis action spectrum to see how light color or wavelength 
affects photosynthesis rate.

To fairly compare the effect on photosynthesis of different light colors 
requires an expensive PAR meter to assure each color provides the same amount 
of light. 

References


Re: Do different colors of light affect the growing rate/ability of plants?


Re: what is meant by an action spectrum as opposed to an absobrtion 
spectrum


Re: How much light does chlorophyl absorb? How much other materials?


Examples of transmission spectrum graphs


Solar Spectrum Graph 


Incandescent (Tungsten) Lamp Spectrum Graph


Fluorescent Lamp Spectrum Graph