Directions for teachers

Before beginning this activity, students should be introduced to light and electromagnetic radiation. They should understand how properties of light, including wavelength, frequency and energy, vary across the electromagnetic spectrum. Check out “Exploring electromagnetic radiation” for a lesson plan.

You should also review how light can interact with objects, which depends on an object’s composition and the wavelength of incident light. These interactions can include reflection, refraction, absorption, transmission, polarization, diffraction and scattering. Fluorescence is an example of absorption: light hits an object, it is absorbed, and then light at a longer wavelength is emitted.

Start with a teacher-led demo on fluorescence. Ask students to read the Science News Explores article “Scientists Say: Fluorescence” and answer the first set of questions individually. Then, have students read the Science News article“Many frogs glow in blue light, and it may be a secret, eerie language” and discuss the second set of questions with a partner. End with an extension question that will allow students to do research on the evolutionary purpose of fluorescence and its application in industry.

Note: If students choose to do the extension, review the differences between bioluminescence and biofluorescence. Biofluorescence happens when an organism absorbs light and emits it at a different wavelength, whereas bioluminescence refers to a chemical or biological reaction that happens within an organism regardless of an external light source.

Teacher-led Demonstration: Glow show

Ask students to name things they know of that “glow” by fluorescing. Fluorescence is one type of luminescence called photoluminescence, or the emission of light caused by the decay of electrons excited by incident light. Luminescence also includes light produced when chemical reactions excite electrons (chemiluminescence). Some examples of luminescence include highlighters, glow-in-the-dark materials, and brightening agents in some laundry detergents.

Use a black light to do a few demos with common fluorescent materials. Consider using turmeric root in water, tonic water with quinine, honey, or liquid laundry detergent. Ask students to describe what they see. Then, write a “hidden message” to your class in a yellow highlighter on a yellow sheet of paper. Use a black light to reveal the message. Ask students to take detailed observations. As an optional assessment at the end of this lesson plan, ask students to explain why they can see the message under ultraviolet light, but can’t see it well in normal light. Encourage students to use their observations to form their hypotheses.

Directions for students

Read the Science News Explores article “Scientists Say: Fluorescence” and answer the first set of questions individually. Then read the Science News article “Many frogs glow in blue light, and it may be a secret, eerie language” and discuss the second set of questions with a partner.

The science behind fluorescence

1. In your own words, what is fluorescence? And what conditions must be met for a fluorescent material to glow?

Fluorescence happens when an atom or molecule absorbs light at one wavelength and then emits light, typically at a longer wavelength. The material only glows when it is exposed to certain wavelengths of incoming light.

2. Given the examples of fluorescent materials from the article, or based on what you saw in the demonstration by your teacher, why do you think different types of fluorescent materials glow differently (with various types of incident light, intensities, color, etc.)?

Different types of materials have different fluorescent molecules within them. Because the molecules vary in their chemical structure, their electrons’ interactions with light vary.

3. Explain what happens to the electrons in a fluorescent atom or molecule when they encounter certain types of electromagnetic radiation or light. How does this behavior relate to your understanding of atomic structure?

When electrons in an atom absorb certain wavelengths of electromagnetic radiation, the electrons are excited from their stable ground state energy level to a higher energy level. As electrons in atoms jump to higher energy levels, the particles’ most probable locations become farther from the nucleus. When the electrons emit the radiation they absorbed, the particles relax to their ground state energy level. This behavior of electrons aligns with electrons existing in discrete energy levels around the nucleus.

4. Draw a simple diagram depicting how light interacts with electrons in a fluorescent material. Make sure you show the external light source and electron energy levels and indicate when light is being absorbed and released by the electrons.

Student answers will vary but should show light being absorbed when an electron moves from a ground energy level to an excited energy level, and light being emitted from the reverse process. Students may note that there have to be additional energy level jumps on the way down, or else the absorbed light would be the same wavelength as the emitted light.

Fluorescing frogs

1. Why are the frogs an example of biofluorescence? Explain the difference between fluorescence and biofluorescence.

The frogs have fluorescent proteins, pigments or other chemicals in their skin. When a living thing gives off light in this manner, it’s called biofluorescence. Frogs are living organisms that fluoresce.

2. Why didn’t researchers who previously studied frog fluorescence know more about the variety of fluorescence seen in different frog species? Based on what you now know, why would their experimental procedures fail to find fluorescence?

Not many frogs were tested for fluorescence under different types of light. Usually, frogs were only tested under UV and violet light. Different fluorescing molecules glow under different incident wavelengths of light. So using many different light sources would increase the likelihood that frogs’ fluorescence would be found.

3. What is a potential evolutionary advantage of this fluorescence in frogs?

Though researchers aren’t clear about the evolutionary advantage of fluorescence in all frogs, biofluorescence may be used as a signal to other animals or as a signal between individuals of the same species during courtship.

Optional extension: If molecules produce characteristic wavelengths of fluorescent light, how could that be useful in scientific research? Research an example of fluorescence that has been applied in science. Describe it and explain its purpose.