Black lights are a fun discovery at any age, and find application for entertainment value as well as their uses in many different sciences. Putting black lights to use in the classroom is easy, and well worth the minimal investment. Descriptions of possible black light activities are presented below, grouped somewhat by how advanced they are, and the sciences to which they are most appropriate.
Before you start any activities, put safety first. Black lights produce ultraviolet (UV) light. While not visible to the eye, UV radiation is high energy light, and can damage the eyes. UV also causes damage to the skin, ranging from mild irritation and sunburn to the potential development of skin cancer. Limit exposure to UV. Do not shine black lights directly onto people, or if you must, do so for as short a time as possible. A light box can be helpful in directing the light only where you want it to go. Wear eye protection. (Standard plastic safety glasses or goggles will block UV light.) Cover exposed skin. Lab coats and gloves may be helpful for this. You might consider wearing a high SPF sun screen to protect faces as well. For a quick demonstration, less protection is needed than for a full blown experiment, but since student safety is your responsibility, it is always best to err on the side of caution.
ACTIVITY #1: Common UV Active Materials
Appropriate for any age, but you will need to tailor the discussion of fluorescence/phosphorescence according to their level.
Students use black lights to examine various everyday materials to see which will “glow” when exposed to UV. This activity can take the form of a scavenger hunt, where groups armed with black lights hunt through the classroom to find items that glow, or it may be more organized, with the teacher wielding the black light and testing objects that students (or the teacher) suggest.
There are many familiar items that will respond to UV. White paper and white T-shirts are usually the first discoveries. Teeth will also glow, but for safety reasons limit this to a minimal exposure. All glow-in-the-dark items will work. Anything in “neon” or “fluorescent” colors will be likely to respond as well. More surprising items may include petroleum jelly, antifreeze, and Mountain Dew. A longer list of UV-active items can be found at About.com.
Having discovered materials that glow, you can proceed to discuss fluorescence and phosphorescence – the two processes that students have just observed. In both cases, the UV light is absorbed by molecules within the materials, exciting electrons to a higher energy level. In the case of fluorescence, the electrons relax (return to ground state) almost immediately, emitting light in the process. Some of the energy is lost by other mechanisms, so the light emitted is lower in energy than the energy that was absorbed. Lower energy light has a longer wavelength, with the result that the light emitted in fluorescence is now visible to the human eye. Phosphorescence works on a similar principle, except that relaxation does not occur immediately. The excited electron reaches a metastable state – an excited energy level that is more stable than having either a little more or less energy, though not as stable as the ground state. The electron stays in this state for a while, before returning to the ground state and emitting longer wavelength light. Objects that continue to glow after the black light has been turned off or moved away are exhibiting phosphorescence.
ACTIVITY #2: Glowing Rocks
Geology or Earth Science classrooms will enjoy this activity.
A number of minerals are known for their UV activity. It is common to simply demonstrate this either in the classroom or in a visit to the natural science museum (where displays of fluorescent minerals are colorful, but placed behind glass). This is good for a couple oohs and ahs, but doesn’t involve much student participation. Instead, give students the opportunity to discover which minerals will fluoresce themselves.
Have students hunt for different types of rocks and crystals at home (inside or out), in parks, at the beach, and wherever they may go. Stockpile these rocks for a week or two, until there is a good variety, then arm students with black lights and let them identify which rocks fluoresce. Have them separate these out. Put them to work sorting the fluorescent rocks by type. They don’t need to identify them at this point; similarity of appearance will be adequate. Once the rocks have been sorted, allow them to attempt identification. You’ll need to provide reference materials for this. You may need to assist as well, depending how much mineralogy you’ve taught them.
After discovering which local rocks fluoresce, go ahead and display a collection of other fluorescent rocks and minerals (fluorite, calcite, opal, gypsum, etc.). You probably already have a collection, but if not, they can be purchased relatively cheaply from educational and suppliers. A good listing of fluorescent minerals is available online from Amethyst Galleries.
With an advanced group of students, you may examine the chemical structures of fluorescent minerals. If you can identify the arrangements that lead to fluorescence, you should then be able to predict whether other minerals will or will not fluoresce.
ACTIVITY #3: Biological Traces
Biology and forensics students will appreciate (or be disgusted by) this activity.
Urine, blood, feces, semen and other biological emanations are known to fluoresce. This means that black lights have found real-world application in everything from crime scene investigation to hunting for stains the family dog may have let on the couch. Cleanliness experts may take a black light with them to motels to verify that sheets have been washed thoroughly. (Don’t try this if you don’t want to know.) Glowing spots make a tell-tale sign that there has been biological activity.
You can show pictures to students, of course, to provide examples. As always though, doing is more memorable than merely looking. You can create a forensic experience for students.
Put together a scenario for some crime (rape, murder, or something less dramatic if your students can’t handle those topics). You’ll need to provide a story as well as the crime scene itself, so make sure to include appropriate details such as victim or witness accounts. This gives students facts to corroborate or disprove with actual evidence. Your crime scene will include carpet or sheets or clothing that does or does not bear stains. Be creative.
Evidence can lead to guilt or innocence of accused parties, and both are valuable. Evidence can also lead to more questions. If the victim was found stabbed and lying on the carpet in a blood-stained shirt, but the carpet is blood-free, the question of where the victim was killed arises. You don’t have to give students the full mystery to solve. Once they’ve completed their part, you can tell them what police went on to find and what the resolution was.
While you might consider using a dog’s urine (if you can get a dog to cooperate), do not use any human material, as the risk of infection is too high to be acceptable in the classroom. Rather than using actual biological material at all, you are probably best off using a solution of a fluorescent dye instead. (See Activity #4 for a list of fluorescent dyes.)
ACTIVITY 4: Art with Fluorescent Chemicals
Good for the chemistry classroom, but also fun for art students.
Fluorescent molecules have many practical uses in scientific research, but it’s normally more fun for students to use them for something creative than to read about the latest research in tracing DNA markers. With fluorescent dyes, you can make glowing pictures, glowing sculptures, and hidden messages.
Prepare solutions (in either alcohol or water, depending on the dye’s properties) of fluorescent dyes. There are many fluorescent chemicals available, but here a few relatively common ones that enable you to cover the color spectrum. The colors listed are the colors they emit when exposed to UV. Don’t be surprised if they look different in solution.
Cy5 [a cyanine dye] (red)
Texas Red (orange)
Phycoerythrin (yellow)
Fluorescein (yellow-green)
Rhodamine green (green)
DAPI (blue)
Tryptophan (purple)
– Glowing Pictures
Students “paint” their picture using the solutions of fluorescent dyes. This may be a challenge, since they can’t see the result until later, but that’s part of the fun. Make sure they know that it doesn’t have to be a perfect work of art, so they can set aside their perfectionism. Abstract art is perfectly acceptable for this exercise, so shapes and lines are fine. Once the pictures dry, hang them up, turn off the lights, and view them with the black light. Students should be pleased with the intensity of color compared to the bland solutions they started with.
Since white paper fluoresces under UV, you may not want to use it for your canvas. Try black paper, newspaper, cardboard and other surfaces instead.
– Hidden Messages
Because the fluorescent dyes aren’t very impressive in daylight, they can hide easily amid more pronounced colors, only to be revealed in the presence of UV. Using normal watercolor paints, you can create a picture that absorbs the eye’s attention. Using one of the fluorescent dyes, write in the secret message. Once dry, you can reveal the message in the dark with a black light.
– Glowing Sculptures
You’ll need narrow glass tubing (preferably quartz glass) and a hot flame to allow you to shape the glass. If you haven’t taught your students the basics of bending glass tubing, you’ll have to do this as demonstration only. Shape the tubing into whatever form you want – it could be a letter, a spiral, or whatever. Using a pipette, fill the tube with a solution of fluorescent dye. Stopper the ends. If you used quartz glass, you can shine the black light directly onto the sculpture in the dark, and the dye will fluoresce. If you used regular borosilicate glass, you’ll need to open one end of the tube and shine the UV light in the end. (Borosilicate glass is opaque to UV light, though it will produce a faint blue fluorescence itself.) A black light will not be very useful in this case – you might want a little UV keychain laser instead.
ACTIVITY #5: UV Really is Light
Applicable to Physics and Physical Science, this activity uses concepts of energy, transparent and opaque materials.
Light is a difficult concept for students at times. They are familiar with the visible spectrum, but anything outside of “ROYGBIV” is harder. How do we know it exists, if we can’t see it? By examining the effects of UV light, students can “see” that it really exists, and that it behaves in similar fashion to the way visible light does.
Connect a photovoltaic cell with a buzzer, a little LED light, or some other indicator. Students should already be familiar with the concept that a photovoltaic cell converts light energy into electrical energy. Remind them, and demonstrate that visible light (the classroom lights or a flashlight) will cause the indicator light or buzzer to activate. Extinguish all lights and shine the black light on the cell. The indicator should again activate. The cell is not picky about the type of light, so long as there is enough energy. UV is more energetic than visible light, so it powers the device just fine.
If you have already introduced filters to your students, they should be aware that a material can be transparent to certain wavelengths of light and opaque to others. This is true with UV as well. Demonstrate this by placing a piece of clear plastic over the photovoltaic cell. The plastic is transparent to visible light, so the classroom lights still cause the indicator to activate. Using the black light in the darkened classroom fails to activate the indicator. Clear plastic, it turns out, is opaque to UV light even while being transparent to visible light. Repeat the experiment using other materials, such as window glass, borosilicate glass (what most laboratory glassware is made from), quartz glass, and other materials that students may suggest. Of the examples listed above, only quartz is transparent to UV light.
