Fluorescence
Light as Energy
Two simple but powerful activities helps students understand
light as energy and apply their understanding to how plants
capture and use sunlight for energy.
Discussion and Background
First described as early as the sixteenth century, fluorescence was documented and so
named in 1852 by George Gabriel Stokes. But it wasn’t until the twentieth century that
the quantum nature of light, and thus fluorescence was finally understood.
Here’s why - by the nineteenth century, Newton’s idea that light was composed of
particles had been replaced by Huygen’s wave theory of light. The wave theory, as with
most classical physics, assumes that change occurs as a continuous process.
Embarrassingly, fluorescence could not be explained by the wave theory of light.
Enter Max Plank, who suggested that energy changes might actually occur in a
stepwise manner, and “jump” from one state to another, rather than proceed as a
continuous process. When Einstein applied this idea to light energy he re-introduced
the idea that light is made of particles (which we now call photons).
Plank then formalized this understanding of light energy by relating the frequency of
light to its energy.
Mathematically:
E= hn Where: E = energy h = a constant, and n = the frequency of light.
The take away here is that the frequency of light is directly related to its energy – and
that the energy of light is quantized, or occurs as definitely defined “steps” or values,
rather than as a continuous flow. It is by understanding light in this way that we can
understand what is happening when something fluoresces.
When light hits a material, the light is either
transmitted or absorbed by the material.
Certain materials are fluorescent, which means
that they contain molecules whose electrons
may become excited – and excited electron
“jumps” energy levels.The electrons in
fluorescent materials become excited when
exposed to light of a high enough frequency
(light with enough energy) to cause the
excitation.
But the excited state is not maintainable for very long – a tiny fraction of a second,
actually – before it falls back down to its original state. When it does this, it releases the
energy it gained in order to “jump” as LIGHT of a different frequency from the original
light.
UV light has a very short wavelength, and therefore has a high frequency and plenty of
energy to fluoresce lots of materials. But some fluorescent materials are sensitive
enough to fluoresce with visible light.
Activity
Materials for each group of 4-5 students
• Set of 3 LASER Blox
• Three fluorescent markers
• A piece of plain white paper
• A pair of rainbow glasses for each student
Procedure
1. Distribute materials to students
2. Review LASER safety guidelines
3. Instruct students to put on the glasses and
look around the room at the “white lights”
4. Explain that the glasses are Diffraction Film
and break the white light up into its
constituent wavelengths so that they see a
spectrum.
5. With the glasses still on, have them turn on
and point the RED LASER Blox at the white
paper to observe the diffraction of
monochromatic light. (for more see our
lesson about the properties of LASER Light)
Note that monochromatic light does not
produce a spectrum of color - it “splits” the
light into a diffraction pattern.
6. Repeat step 5 with the GREEN LASER Blox
7. Repeat step 6 with the VIOLET LASER Blox - and notice the spectrum! While the
green and the red LASER Blox are not of sufficient energy to cause fluorescence
upon hitting the white paper, the VIOLET LASER Blox is! The spectrum is caused
not by the diffraction grating, but by the light’s interaction with the molecules in the
paper. The energy of the 405nm violet laser light “knocks” electrons out of orbit when they fall back down, they release energy in the form of light. The released light
is not 405nm -- It is released at several other wavelengths. The diffraction grating
allows us to see the spectrum of wavelengths released by the phenomenon of
fluorescence! See background for a more thorough explanation.
8. Now color three squares with fluorescent markers.
9. Start with the RED LASER Blox - with the glasses still on, point the beam at each
square. What do you see? Does it fluoresce on the fluorescent colors? NO. Because
the red laser, at 635nm, does not generate enough energy to cause the electrons to
“jump” out of orbit.
10.Now try the GREEN LASER Blox. Notice that it fluoresces with the pink and orange,
but not with the yellow. Discuss.
11. Finally try with the VIOLET LASER Blox. Discuss.
Diffraction of a Green Laser
Diffraction reveals the
spectrum created by 405nm
laser on white paper!
Fluorescence spectrum with
532nm laser on fluorescent
marker.
Activity 2
Now that students understand fluorescence, you can use the phenomenon to learn
more about how plants capture and convert energy from the sun.
One of the most common fluorescent molecules in
nature is chlorophyll, the green pigment found in
plants. Leaves are green when lit by the Sun
because they reflect the green light wavelengths.
Chlorophyll absorbs mostly blue and red light
wavelengths, which are the driving energy of
photosynthesis. The energy of light is measured as
wavelength: the shorter the wavelength, the
stronger the energy of the light. When illuminated
by ultraviolet light, the plant pigment glows red,
which is lower in energy. The missing energy or
difference in energy between the high-energy of
the UV light and the lower-energy red light is
released as heat energy.
In this activity, we’ll extract chlorophyll from the leaves of spinach and see that it
fluoresces RED when exposed to UV or near UV light - and find out what this teaches
us about how plants gather and store energy from sunlight.
Materials for each group of 4-5 students
• Mortar and Pestal
• 6-8 fresh spinach leaves
• ~ 50ml 100% ethanol
• Set of LASER Blox
Procedure
1. Place spinach in mortar and crush with pestal
2. Pour ethanol on crushed spinach and stir
3. With Lights dimmed, point the violet Laser Blox at the mixture
Discussion
What does chlorophyll do with the light energy it has absorbed if the light energy
cannot be converted to chemical energy?
In the intact leaves of green plants absorb photons in the blue/violet range of the visible
spectrum. Electrons excited by light enter the electron transport chain within the
membrane of the thylakoid, and are used to synthesize sugar for the plant.
Crushing the spinach leaves and adding ethanol isolates chlorophyll from the thylakoid
membranes of the chloroplasts. Freed from the electron transport chain, the electrons
release their energy in the form of red light.