Fluorescence of Everyday Items

Introduction

Turn down the lights and wow your students with all the everyday items that glow under ultraviolet light.

Concepts

• Fluorescence
• Ultraviolet light

Background

Luminescence is the emission of radiation (light) by a substance as a result of absorption of energy from photons, charged particles or chemical change. It is a general term that includes fluorescence, phosphorescence and chemiluminescence, to name just a few special types. Fluorescence is different from other types of luminescence in that it is restricted to phenomena in which the time interval between absorption and emission of energy is extremely short. Therefore, fluorescence only occurs in the presence of the exciting source. This is different from phosphorescence, which continues after the exciting source that has been removed.

In fluorescence, when a light source is shined on a material, a photon is absorbed. The energy from the photon is transferred to an electron that makes a transition to an excited electronic state. From this excited electronic state the electron naturally wants to relax back down to ground state. When it relaxes back down to ground state, it emits a photon (symbolized by the squiggly\ arrow in the diagram). This relaxation may occur in a single step or in a series of steps. If it occurs in a single step, the emitted photon will be the same wavelength as the exciting photon. If the relaxation occurs in a series of steps emitting a photon along the way, the emitted photon will have a greater wavelength (lower energy) then the exciting photon.

If the emitted photon’s wavelength is in the visible portion of the spectrum, we observe a colorful, glowing effect. Emission of the form is termed fluorescence. This process is practically instantaneous so that fluorescence is observed as the exciting source is present, and it disappears as soon as the exciting source is removed.

Materials

Safety Precautions

The materials used in this lab are considered nonhazardous. However, all items brought into the laboratory should be treated as potentially hazardous and no longer used for their intended purpose. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Dispose of excess materials in the trash according to Flinn Suggested Disposal Method #26b.

Procedure

Part A. Pyramid

1. Start this demonstration by building a big pyramid of fluorescent packages at the front of the room and have the blacklight at the bottom of the pyramid.
2. Then turn off the room lights and turn on the blacklight.
3. Explain that the blacklight is mainly putting out light that is just past their range of vision so they see it as “dark.” The light is absorbed by the paints on the boxes and then emitted in various regions of the visible spectrum.

Part B. Whiteners

4. Tell the students that many times manufacturers put “whiteners” in their products that make them appear to clean better. The “whiteners” really are fluorescent substances that will cling to the object that is being cleaned. They not only reflect the light that hits them but also emit more light in the visible spectrum.
5. Laundry detergents are a product that shows this nicely. Make small piles of various types of detergent and then shine the blacklight on them. The fluorescent particles in the detergent really stand out.
6. Make a slurry of the detergents and it will fluoresce very well also. Tide brand always works well. The liquid Tide^® works especially well! Under normal light it doesn’t appear that it would fluoresce at all, in fact is looks clear, but if it is spread on a piece of paper and placed under the blacklight it fluoresces a nice blue.
7. Write “invisible” messages on paper and under blacklight they appear!

Part C. Toothpaste and Shampoo

8. Many other household substances also fluoresce nicely. A few days before this class, wash your hair with Silver Fox^® shampoo and brush your teeth with Sensodine^® toothpaste. When the blacklight is shown on your hair and teeth they also fluoresce!
9. Vaseline is also a very fluorescent substance. Write a message on a white board with it and, just like the liquid Tide, it will fluoresce! This seems to amaze the students.

References

Special thanks to Rhonda Reist of Olathe North High School, Olathe, KS, for providing us with this activity.