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How do different color filters affect plant growth?
Question Date: 2000-01-24
Answer 1:

Your question is a very good one becauseplants absorb light at very specific wavelengths to obtain energy.

In particular, chlorophyll absorbs blue and red light while allowing green light to be reflected (or transmitted). This is why plants appear to be green to us.

If chlorophyll needs red and blue light, what do you think would happen to the plant if you were to place a green filter over your light source so that the plants didn't get any blue or red light? Would the plants be ok if you used a red or blue filter and blocked out the green and blue or red light?

Answer 2:

This is a classic question that many grade school science projects have attempted to answer for many many years. The answer has to do with an understanding of the rainbow (or the full visible light spectrum) and the color of plants' leaves.

Leaves appear green because they reflect green, while absorbing all other colors of light. Think of the rainbow, what colors are there other than green? Well, there's red at one end of the spectrum and blue or violet at the other. A rainbow represents all the colors that the sun emits and that get through the Earth's atmosphere. What we perceive as white light is actually a mix of all the colors in the rainbow.

So all those colors are shining down on a plant's leaves and the plant is absorbing all but the green. Generally you can say that plants absorb primarily red (or red/orange) and blue light. It's within the chloroplasts that all this light absorbing happens.

The chloroplasts take the energy harnessed in these light rays and use it to make sugars for the plant to use in building more plant material = photosynthesis. Within the chloroplasts, the molecules that actually do the absorbing are called photopigments (freckles are examples of pigments in humans).

A plant has a mix of different types of photopigments so that it can absorb light at different colors. A plant can have one photopigment devoted to absorbing deep blue, another devoted to absorbing yellow, another for orange, and another for red. When full spectrum light, like sunlight shines on a plant all the photopigments are activated and absorb their "specialty" color. A plant's chloroplasts get all the actions of the photopigments coordinated so that they're all working to harness most of the sun's light rays and make plant food.

If there is only one color of light shining on a plant, then only a certain group of photopigments are active. The plant won't be able to make as much sugar or plant food as when there is full spectrum light shining on it and it may suffer generally. Not only will the plant not have enough light to make lots of food, but the plant uses these different color lights to signal all sorts of other internal processes. If, for example, only blue light was shining on the plant, then all the red-light triggered processes would not occur.

Eventually the plant may die because of this lack of full spectrum light and certain processes not happening. It would be like in your body if suddenly your liver couldn't function anymore. Eventually you would die. So, plants need full spectrum (all the colors of the rainbow) light to live productively.

Here's a question for you, since now you know why a plant's leaves are green when the plant is alive and healthy, what is happening in the autumn, when the leaves turn yellow, red and orange just before falling off?

Answer 3:

This is a question my college students often ask. You know that "white light" like what you get from a bulb is made of different colors, right? If not, prove it to yourself using a prism.

Filters only allow one color (or set of colors) pass through them. If you use a green filter, what color light goes through? (Test it yourself with a filter and light source.)

We see things as a certain color because when white light shines on an object, some colors (wavelengths) of light are absorbed. Others bounce back off the surface into our eye. Imagine you are throwing small foam balls at a wall. Some are absorbed into the wall; you never see them. Others bounce back and hit you in the eye. With light, you only see the light that bounces back and "hits you in the eye". If the wall looks blue, which colors (wavelengths) are bouncing back off the wall? All of the other colors are being absorbed. If a plant is green, what colors are being absorbed? What color is bouncing off?

Now imagine that you are throwing little balls of energy at a plant. Only the ones that stick are used by the plant. The energy balls that bounce off do the plant no good. The plant actually gets its energy from light. It can't use light that "bounces off", only light that it absorbs. So which colors of light actually provide energy to the plant? Which color bounces off without providing energy?

Now you are ready to answer your own question, which filters will allow a plant to get the least energy and grow the slowest? Hint: What color light bounces uselessly off the plant? Among the colors that "stick", which have the highest energy (shortest wavelength)?

If you decide to test your hypotheses with colored lightbulbs or plastic wrap, have an adult help you set up your experiment safely.

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