Re: Distorted results in experiment with artificial light

Dear Tiffany - 

     Your problem may be in the filters you are using.  They will transmit 
different intensities of light, as well as different wavelengths.  You 
might find that everything looks a lot darker through some of the filters 
compared with the others.  To do this experiment well, you need filters of 
equal density - but these are usually very expensive!

     I'm not sure whether you're interested in the germination or the 
growth of the plants, so here's a basic explanation for both.

     Plants are dependent on light for growth, and for some plants there is 
no point germinating if there's no light around.  The seeds of these plants 
have clever detectors which are able to tell if they're in the dark or 
light.  These detectors are switched on best by red light, and aren't 
switched on at all by blue light.  You can see this if you grow some 
lettuce seeds on wet tissue, and grow them either in the dark, the light, 
under a red filter, or under a blue filter.  You should see only a few 
seeds germinating in the dark and lots germinating in the light.  You 
should see lots germinating under the red filter (as the red light 
detectors in the plant are switched on), and only a few germinating under 
the blue filter (no red light, so the red light detectors aren't switched 
on, and the plant thinks it's still in the dark).  This works really well 
with lettuce seeds, and I'm not sure how well it works with beans.

     Once they've germinated, the young plants will be growing as hard as 
they can.  To do this they need lots of light (and the right sort of 
light).  Green light is no use to plants - they are green because they 
reflect all of the green light shone on them.  They can only use red and 
blue light, because of the way chlorophyll in the plant absorbs the light.
For the same intensity light, plants grow better on red light than blue, 
and not very well on green light.

For lots more information on photosynthesis (in great detail) see this
Photosynthesis guide

      There are two important experimental things to remember as well.

1)  How do you measure growth in your plants?  A plant kept in the dark 
might look as though it is growing very well, as it's shoots become very 
long and tall.  After a while though, it will die as it can't get any food 
(sunlight!), so really it is not growing very well.  The way to measure 
growth best in plants is to measure the weight of a plant once it has been 
dried in an oven, as this tells you how much plant the plant has built.  
Something to think about - what is the best way to measure growth (weight 
or height?).  Ask your teacher about it.

2)  There is a lot of variability in biology - a plant may do the 
completely opposite thing from one experiment to another.  The way to get 
round this is to look at lots of plants for each treatment, and get an 
average for each treatment.

Hope this helps!   All the best,  Carl.