|How do different color filters affect plant growth?
|Question Date: 2000-01-24|
Your question is a very good one because plants
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?
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 amix
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.
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,
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
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
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?
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. A good
picture of the light spectrum is available at:
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
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
Among the colors that "stick", which
have the highest energy (shortest wavelength)?
Check the site I listed above or look in a physics
book or encyclopedia. Which will give more energy
for plant growth, low energy light, or high energy
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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