Measure Luminescence in Glow-in-the-Dark Objects

Abstract

Objects that glow in the dark hold a special place in the imagination of both children and adults. The lights go out at night, but these odd things refuse to disappear. Where does the light come from? Do they work in any climate? In this science fair project, you will build an electronic device that measures the light given off by luminescent materials. The device will be used to study how temperature affects luminescence.

Objective

Build a device to measure the intensity and duration of light produced by glow sticks and other glow-in-the-dark materials. Determine how temperature affects the intensity and duration of luminescence.

Credits

David Whyte, PhD, Science Buddies

Last edit date: 2013-01-10

Introduction

Helium balloons, soap bubbles, and glow-in-the-dark objects seem to defy the rules that govern most everyday things. In the case of bubbles and balloons, they go up, but float around before coming back down. And glow-in-the-dark objects produce light without an apparent source of energy. This science fair project focuses on two types of glow-in-the-dark phenomena: phosphorescence and chemiluminescence.

Phosphorescent objects don't use batteries to glow. In fact, they behave like batteries themselves—you "charge" them up with light, and they then provide a steady source of light for hours after. The basis for this afterglow is the unique way that the molecules in phosphorescent materials react to light. Chemiluminescent objects, such as glow sticks, use chemical energy to produce light.

When you shine a light on an object, such as a glow-in-the-dark toy, you are bombarding it with photons. Photons are what light is made of—small packets of energy that have zero mass and travel at the speed of light. When a stream of photons collides with a solid material, the photons interact with the electrons in the material. Electrons that absorb photons move to a higher energy level. Eventually, the electrons return to the lower "normal" energy level, called the ground state. When the electrons moves to the ground state, the energy stored in each electron is released as a photon (light).

If an object absorbs all of the colors in a stream of light except red, then red light bounces off of the object and you see it as red. The interplay of photons with electrons determines which colors are absorbed and which are not. For most objects, the interactions between the photons and the electrons cease when the light stream is turned off. In the case of a phosphorescent material, some of the electrons that interacted with the photons become trapped in a meta-stable state. These electrons have absorbed energy from the photons, which moves them into a higher energy state, but they are not able to release the energy right away. The term meta-stable implies that they are in a precarious situation —they could be knocked back down to their normal energy level with just a little jostling. An electron in a meta-stable state is like a Frisbee or a ball caught in the branches of a tree—it is temporarily resisting the pull to a lower state, but a little shaking will bring it back down.

In the case of an electron in a meta-stable state, in a phosphorescent material, the "shaking" is caused by heat. The higher the temperature of the object is, the more energetic the motion of the molecules in the object will be. The molecules can bump into each other, and if a meta-stable electron is bumped just right, it will fall back to its normal energy level. The electron might stay in its meta-stable state for seconds, hours, or even days, but on average, the length of the meta-stable state decreases at higher temperatures. When the electron is eventually jostled down from its meta-stable state, it gives up the energy it possessed as a photon. The glow you see in a phosphorescent object is due to the release of energy, as photons from meta-stable electrons return to a lower energy level.

Chemiluminescence also produces glow-in-the-dark "magic," but by a different mechanism than that of phosphorescence. When you twist or bend a glow stick, you start a chemical reaction. One of the products of the reaction is light. There are three components of a glow stick: a solution of hydrogen peroxide, a solution of a phenyl oxalate ester, and a fluorescent dye. The basic premise of the reaction is that the reaction between the two chemicals releases enough energy to excite the electrons in the fluorescent dye. This causes the electrons to jump to a higher energy level and then fall back down and release light. Depending on the chemicals used, a variety of different colors can be produced.

Although you could try to do a science fair project by just looking at the glowing material, it is hard to get good numbers that describe the results using visual methods—accuracy and measurable results in experimentation is important. The Experimental Procedure, below, describes a device that will allow you to measure the amount of light that is given off by luminescent objects. You will use a kit from RadioShack to build a light detector—it is easy to assemble and the circuits are just what you need to sense and measure light.

The circuit you will make is basic, with the key component being a photoresistor. A resistor, as its name suggests, inhibits the flow of electrons in a circuit. The level of resistance is measured in ohms (Ω), and it is usually a fixed quantity. In a photoresistor, the level of resistance is altered by light. As the intensity of light increases, the resistance in the photoresistor decreases. This is handy if you want to measure the level of light. With a simple circuit containing a photoresistor and a battery, the amount of light can be measured by tracking changes in voltage. You can build the circuit from parts obtained at an electronics store. Alternatively, you can buy an electronics kit that contains the parts you need (recommended). The kit will make it easier to set up your measuring device and will also allow you to swap various components to change the sensitivity.

To isolate the luminescent material from the light in the room, the Experimental Procedure calls for it to be placed in or on (for paper) a glass jar, and then covering the jar with tin foil. The photoresistor is placed in a hole in the jar top and covered with black tape to block the surrounding light. This setup allows you to explore various materials and variables, such as time and temperature. Although the circuit is fairly straightforward, especially if you use the kit, this is a DIY ("do-it-yourself") project that will call for some creative problem solving on your part. But once you have it worked out, you can describe, with precise numbers, what you see when you are looking at glow-in-the-dark objects.

Terms and Concepts

• Phosphorescence
• Chemiluminescence
• Photon
• Electron
• Energy level
• Ground state
• Meta-stable state
• Photo-resistor
• Resistor
• Resistance
• Ohm
• Voltage drop
• Potentiometer

Questions

• What kinds of chemicals or minerals are phosphorescent?
• What are some similarities and differences between chemiluminescence and phosphorescence?
• How would you expect temperature to affect the length of time a phosphorescent object glows after being exposed to light?
• What will happen if you put a glow stick in hot water? In other words, how does increased heat affect the rate of a chemical reaction, such as the one in a glow stick? Hint: Look up the Arrhenius equation.
• How does a photoresistor work?
• The circuit for this science fair project measures voltage changes caused by changes in resistance. How are voltage and resistance related to each other? Hint: What do the terms in the equation V=IR stand for?

Bibliography

• Helmenstine, Anne Marie, Ph.D. (2008). How Do Lightsticks Work? Retrieved November 4, 2008, from http://chemistry.about.com/od/howthingsworkfaqs/a/howlightsticks.htm
• Schmidt, W. (2008). Super-bright glow-in-the-dark paints. Retrieved November 3, 2008, from http://www.waynesthisandthat.com/glow.htm
• Glow, Inc. (2008). Glow! Retrieved November 3, 2008, from http://www.glowinc.com
• Nave, R. (2008). Quantum Processes. Retrieved November 3, 2008, from http://hyperphysics.phy-astr.gsu.edu/Hbase/mod5.html

Materials and Equipment

• Electronic Sensorslab, from RadioShack, part # 280-0278
• Glass jars with metal lids, 24-oz. (3 matching)
• 9-volt (V) battery
• Glow sticks (10), small enough to fit easily in the glass jar; available at most toy stores or online
• Phosphorescent paint or paper (at least 2 different types of either paper or paint); available at hobby shops, toy stores, or online from the sites listed in the Bibliography
• Tin foil
• Black electrical tape
• Drill with 1/4-inch drill bit to make a hole in the jar lid. A stiff knife will work, too.
• Flashlight, as bright as possible
• Liquid measuring cup
• Infrared thermometer; available at local home goods stores or online from retailers such as Amazon.com
• Lab notebook
• Graph paper
• Helper to record data
• Optional: To get a more precise measurement of the light intensity, use an autoscaling digital multimeter for the readout. For example, the Equus 3320 Auto-Ranging Digital Multimeter, available at Amazon.com. Set the multimeter to read DC (direct current) volts. Attach the two leads from the multimeter to the same output wires that go to the LED readout (springs 19 and 20).

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Experimental Procedure

This science fair project is based on the Sensorslab kit from RadioShack. The kit has more components than you need, but it will get you up and running quickly. The circuit is one of several described in the Sensorslab kit from RadioShack. It is shown on page 85 of the workbook that comes with the Sensorslab Kit, and is shown below. If you prefer to build the circuit on a breadboard, see the Science Buddies Project What is in this Water? Experiments with a Homemade Turbidity Meter for a description of the procedure.

Building the Light Sensor

The first part of the experimental procedure is to build a light sensor that will allow you to measure how much light is being produced by the luminescent material.

1. Open the Sensorslab kit and set out the parts.
2. Study the first few pages of the workbook to learn the symbols for the circuit parts.
3. The basic light-meter circuit contains a photoresistor, a potentiometer, a battery, and a readout device.
4. To build the circuit, follow the directions in the workbook for the project entitled "Build a simple photoresistor light meter" on page 85.
   1. The kit makes it easy to build the circuit, since the wires are attached using springs (no soldering) and all of the connections are numbered.
5. The potentiometer is a knob that turns to adjust sensitivity. For example, you will want a different sensitivity when you are measuring bright light levels versus very dim light levels.
6. Use the extension cable to attach the photoresistor to the circuit.
7. Once you have built the circuit, test it with various light levels. The readout that is incorporated in the kit has a set of LEDs that provide a 1–10 scale for light intensity.

Figure 1. Photoresistor light meter circuit. The circuit has a 9-V battery, a photoresistor, a potentiometer (the "R" with the crooked line), and a "Readout" meter. The resistance of the photoresistor decreases in the presence of light. The potentiometer, which has a knob that changes its resistance, is used to vary the sensitivity of the meter. In bright light, the voltage drop across the photoresistor is reduced, leading to an increase in the voltage drop across the potentiometer, which is what is measured by the "readout."

Constructing the Light-protected Jar to Test Chemiluminescence

1. The jar should be tall enough to hold the glow sticks.
2. Wrap the sides of the jar in tin foil so that no light can get in.
3. Seal the edges securely with electrical tape.

Constructing the Phosphorescent Material Holding Device

Depending on which method you are using, either follow step 1 or step 2, below.

1. If you are using phosphorescent paper:
   1. Trace and cut a circle out of the phosphorescent paper that is the size of the base of the jar.
   2. Tape the phosphorescent paper circle to the outside of another jar base.
2. If you are using phosphorescent paint:
   1. Paint the bottom of another jar with the phosphorescent paint and let it dry.

Assembling the Light Detector Top

1. Put on your safety goggles and drill a hole in the center of just one of the jar lids, about 1/4 inch in diameter.
2. Tape the photoresistor from the circuit over the hole in the jar top. Use black electrical tape, making sure light cannot enter the jar.
3. This lid will be used on all of the jars.

You have just assembled the light meter and the "housings" for the light sources.

Figure 2. This is the experimental setup to measure luminescent light. The tin foil around the jar protects the sample from surrounding light. See the "Optional" note in the Materials & Equipment section, above, for instructions how to hook up the digital autoscaling multimeter.

Familiarizing Yourself with the Light Detector

It is important that you become very familiar with how the light detector works. Experiment with the potentiometer settings in different lighting conditions.

1. Determine the voltage baseline for the jars with nothing in them and with the lid on. There should be no signal.
2. The light level in the light-protected (tin-foil wrapped) jar should be the same when the jar is in a lighted room or in a dim room. If it changes, that means there is a light leak. Find the problem area and cover the light leak with electrical tape.

Testing the Effect of Increased Temperature on Chemiluminescence

Important Notes:

• The position of the glow sticks in the jar should be the same for each trial.
• Keep movement of the setup to a minimum to reduce unwanted variations.
• Work with a helper so that you can record the readouts quickly.
• Vary the times given below, if desired, to obtain further data.

1. Twist a glow stick to start the reaction.
2. Place the glow stick in the tin-foil wrapped jar and place the lid on.
3. Watch the readout. It should go from a high level at first, and gradually to a lower level as the signal decays. Does it decrease evenly, or is it faster in the beginning? Record your observations in your lab notebook.
4. Adjust the sensitivity, as needed, by turning the knob on the potentiometer.
   1. Note: Once the sensitivity is set, don't change it during the course of the experiment or you won't be able to compare samples.
5. Record the setting of the potentiometer in your lab notebook.
6. Record the time and signal strength produced by the light of the glow stick.
7. Describe how the signal changes with time. What will happen if the temperature is increased?
8. After 15 minutes, open the jar and add hot water to raise the temperature, as follows:
   1. First use the infrared thermometer to record the temperature before adding the hot water. Record all readings in your lab notebook.
   2. Add a 1/4 cup of hot water, should be approximately 50°C (112°F).
   3. Close the jar.
   4. Take the temperature after you've put the lid on the jar.
9. Continue recording the signal strength.
10. Repeat steps 1–9 several times with fresh glow sticks and a new 1/4 cup of hot water (pour out the old water) until you have clear data showing the following:
    1. How the light signal decays with time.
    2. How temperature affects the light intensity.
11. Graph the data.

Testing the Effect of Increased Temperature on Phosphorescence

1. Use the infrared thermometer to record the temperature of the jar with phosphorescent paper or paint, at room temperature.
2. Shine the flashlight on the phosphorescent material for 60 seconds.
3. Put the lid on the jar with the photoresistor attached.
4. Watch the readout. It should go from a high level to a lower level. Does it decrease evenly, or is it faster in the beginning?
5. Repeat steps 1–4 again, but use the stopwatch to time how long it takes for the light to decrease at each level—from 10 to 9, etc.
6. Use the infrared thermometer to record the temperature of the jar with phosphorescent paper or paint, at room temperature.
7. Now add 1/4 cup of hot tap water, approximately 50°C (112°F), to the phosphorescent jar.
8. Shine the flashlight on the phosphorescent material for 60 seconds, as before. Do not put the lid on yet.
9. Measure the temperature with the infrared thermometer.
10. Now put on the photoresistor lid.
11. Record the light level reading and corresponding times in your lab notebook.
12. Repeat steps 7–11 several times (with new hot water—pour out the old water) until you get consistent results.
13. Graph the data.
14. Repeat steps 1–13 with the other type of phosphorescent paper or paint.

Variations

• Try using ice to investigate cold-temperature effects.
• Devise methods for keeping the temperature steady during the experiment.
• Build and test the super-sensitive light-sensing circuits described in the Sensorslab workbook (pages 86 and 91).

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