Bio A – Energy
The Science of Light Energy and Color
Energy comes in many different forms. Light is one form of energy that is particularly important to living things.
Without the light energy from the sun, life on earth would not exist.
The energy from the sun spans a wide range or spectrum called the electromagnetic spectrum. The EM spectrum is
shown below. It includes everything from radio waves to Gamma rays.
Light energy is measured in wavelength which is exactly what it sounds like: the length of a wave of light, from one peak
to the next. Depending on wavelength, the amount of energy in the wave is different.
Your eyes and the light absorbing pigments in plants have something in common. Both can only detect a small range of
energy that comes from the sun. This range is called visible light and it spans waves that are about 380 nanometers
(nm) long to about 750nm long. Light outside this range cannot be detected by your eyes nor can it be absorbed by the
pigments in a plant for photosynthesis. So, in other words, even though UV light can hurt us, we cannot see it. Likewise,
a plant grown in infrared light would be the similar to growing the plant in the dark.
The different wavelengths within the visible light range will all appear to us as different colors. Light in the 380nm range
appears violet and light in the 750 range appears red. The remaining colors of the rainbow fill in the wavelengths in the
middle of the visible light range.
Now for the fun part. What color is your shirt? Easy enough question, right? In reality your shirt only appears a certain
color because of the light that is bouncing off it. Here’s how it works:
-
When light hits an object one of three things happens:
o The light is absorbed by the object. Light that is absorbed cannot be seen by our eyes.
o The light is transmitted, or passes, through the object
o The light is reflected, meaning the light bounces off the object and hits your eye. The wavelength of the
light that bounces off will determine the color of the object.
Bio A – Energy
Different wavelengths of light can combine to form different colors. A light that emits (gives off) wavelengths of 400nm
(blue) and 600nm (yellow) will look green because the two lights mix together. The sun emits all wavelengths in the
visible light spectrum. They all combine together and look white. But REALLY they are many different colors of light
together. If the sun’s light shines through a prism or a raindrop, each wavelength is bent at a slight different angle,
separating them all out so we can see them individually. This is what makes a rainbow!
So back to that shirt. Since you are sitting in a room with white light, your shirt is being hit with all colors of the rainbow.
Depending on the material and dyes used to make your shirt, some light might be reflected. This is what we are seeing.
In today’s activity we’re going to examine a few different types of light to see what wavelengths are really being
emitted. Look at each type of light through the spectroscope and draw the bands underneath the correct place on the
scale using a colored pencil of the right color. Be sure to draw your bands the approximate thickness that you see them.
Use the scale inside the spectroscope to help you measure.
Fluorescent Room lights:
“Daylight” light bulb:
___________ colored light bulb (fill in the color set up by your teacher):
Bio A – Energy
Black light bulb:
Sun (if it is out… Don’t stare too long or directly at it!)
Discussion Questions:
1. Create a table showing the differences in the spectra for the various light sources.
2. What is the difference between the different spectra from the various light bulbs? How did they compare to the
light spectrum of the sun?
3. The colors in the light spectrum have different wavelengths. The shorter the wavelength, the greater the
energy. Which color in the spectrum has the most energy and which has the least amount of energy?
4. Besides light, what are other types of energy that make up the electromagnetic spectrum?
5. Can plants grow with other types of energy from the electromagnetic spectrum, such as microwave radiation if
they do not receive energy in the visible light spectrum? Why or why not?
6. When an object appears a certain color, the object is reflecting those wavelengths of light and absorbing the
colors that you do not see. What is the main color of the light spectrum that most plants reflect?
7. The sun emits white light. Explain what white light really is made up from.
Bio A – Energy
8. You are walking down the sidewalk in bare feet during a sunny summer day. The light colored concrete is warm,
but tolerable. You get to an intersection and step into the black asphalt street. Immediately you start hopping
around like a cartoon character as smoke rises from your burning feet. Why was the asphalt so much hotter
than the concrete when both were in the sun?
9. A similar graph can be made for pigments BUT instead of showing the light energy being emitted (pigments are
not lights!) they show what wavelengths of light and how much of each a pigment can absorb. Below is a graph
showing the percentage absorption of different wavelengths by chlorophyll a. (they were bad and didn’t label
their Y axis but it should say “% absorbed”)
-
If we only had single color bulbs to grow our plants, what wavelength would we want it to be and why?
-
Which wavelength bulb would basically be like growing the plant in the dark? Why?
10. Describe what you think would happen if you grew a plant in EACH of the light sources that you looked at in this
lab. In which light source would the plant grow the best? Why? EXPLAIN!!!