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?
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 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
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,
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
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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