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Leaves and Light

Postby dtjank » Sun Jan 06, 2019 10:01 pm

Leaves and Light is a project idea directly from this site, Science Buddies. The experiment involves covering leaves of a plant with red, blue, green, yellow, and clear cellophane and also some leaves with black construction paper. The experiment uses just one plant. The purpose is to see if various colors of light or lack of light effect the color and appearance of the leaf over the course of one week. In previous ASK AN EXPERT posts and when searching online, we could not find any reports of success with this experiment. We even found resources saying that colored cellophane was ineffective in replicating colored lights. Even for this particular experiment, we found multiple experimenters in the ASK AN EXPERT section and in other sources that reported no difference at the end of the experiment. One possible explanation included in the ASK AN EXPERT section was that the plant used was too adaptable to less light (philodendron) and so this explained why the experiment found no difference in any of the colors including the black construction paper. The explanation was that the leaves of the plant that were receiving light were able to provide the food needed to compensate for the leaves that did not. A suggested solution was that different plants for each color should be used and that all leaves should be covered on the plant. We question how this would make a difference. An added suggestion to this same idea was to also put all of the plants in very low light to compensate for the type of plant. We researched other ideas for plant species but could not find a plant with large enough leaves to allow one to cover each one that requires more light than the philodendron. My son is very discouraged because he already committed to doing this experiment at school but his research is leading him to think his project is not an effective procedure before he even begins. Any suggestions to make this an effective project so he could actually do research that answers a hypothesis about leaves and color? Based on his research his hypothesis would be that the clear cellophane and green cellophane would yield the healthiest plants or leaves because these colors would not block or reflect the other colors of visible light that are important for plant growth. He would prefer to do that since that is the purpose of the experiment. He would rather have an obtainable goal than just knowing already that he will end up researching explanations for his Conclusion of why his hypothesis did not work due to the faulty features of the experiment before the experiment even begins. I know the standard answer is that there is no such thing as a failed experiment but he at least wants to feel like he is not doing an already flawed process before it begins. Any suggestions that would increase his chances for success?

https://www.sciencebuddies.org/science- ... background

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Re: Leaves and Light

Postby lynnsamuelson » Fri Jan 11, 2019 8:43 pm

Interesting that the forums suggest this is a flawed procedure. Colored material should filter out colored light, but from your research, cellophane is not the best material. Based on your research, I would suggest a couple of modifications to the experiment.

First, do extra of the black paper where no light gets in. At the very least, you can have the contrast between full and no light.

Second, you could try to use color filters instead of/ with the colored cellophane. There are several ones, but they run about $10 for a set. Since they are typically pieces of plastic you may need to get creative as to how to place them on the plant leaf without weighing it down too much.

I hope this help and that your child does not get too discouraged by any experiment.

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Re: Leaves and Light

Postby SciB » Fri Jan 11, 2019 9:34 pm

Thanks for the detailed description of the problem. I am sorry that your son is discouraged by this, but do look at it as an experience in learning because there is an explanation there and opportunity for study.

Part of the trouble I think is confusion over the colors of light absorbed by the filters vs the colors transmitted, and the colors of light absorbed by leaves.

When you see green light coming through the filter that means the red and blue wavelengths have been absorbed. Likewise, when you see green light reflected from a plant's leaves it means that the leaves have absorbed wavelengths in the red region and in the blue region. If you put a green filter over the leaf you have reduced the amount of red and blue light and the plant may make more chlorophyll in the leaves in an effort to harvest more light.

Leaves have two types of chlorophyll called, a and b. The two types absorb light in the blue part of the spectrum and also in the red part, but not in the green, which is why the leaves appear green.

Here's a good description of the types of chlorophyll and how they operate in photosynthesis: http://www.webexhibits.org/causesofcolor/7A.html

As to the science project, the hypothesis seems to be that altering the color of the light reaching a plant's leaves will change its appearance in some way. I question whether one week is long enough to see any effect, however. Also, the light coming through the plastic is not absorbed by the leaf if it is green. Only red and blue light are absorbed. That is why a leaf looks green.

The only changes in the leaf that you could be observing would be in pigmentation--the amount of chlorophyll present. Plants respond to low light conditions by making more chlorophyll, but intense light can cause chlorophyll to be degraded. A green filter blocks red and blue so plants think they are being deprived of light and make more chlorophyll. The light that comes through red or blue filters is the wavelength that plants need so the chlorophyll levels should not change.

In one week's time, however, I doubt if you could perceive a difference in the pigmentation of the leaves covered with a green filter compared to those covered with a red filter. With an instrument called a reflectance spectrometer you could probably measure the difference, however.

To reduce the amount of chlorophyll would require that the plants be exposed to intense red and blue light for several weeks. I don't know how intense the light would have to be to degrade the chlorophyll or how long it would take.

My suggestion would be to repeat the experiment and leave the sleeves on for several weeks, observing the leaves every week. Also, the plant needs to be exposed to bright sunlight for at least 6 hours a day.

I think that a smartphone's camera could be used to measure the light reflected from the leaf's surface, but I'm not sure how to do that--with what app. You could try searching for this and see if you can find a way to measure the color of the leaf--the light reflected to your eye. If you can do that accurately and reproducibly then you will have solved the problem of getting data from your experiment.

Please post again later and let us know how the experiment turned out. If you do find a smartphone app that lets you measure reflectance accurately, we would like to know, as this would be a useful tool for many experiments.

Good luck!


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