Introduction: (Initial Observation)
Fluorescent lamps are the most commonly used commercial light source in North America. In fact, fluorescent lamps illuminate 71% of the commercial space in the United States. Their popularity can be attributed to their relatively high efficacy and long operating life. Unlike regular incandescent light bulbs, fluorescent bulbs have no light producing filament, so what is there that emits light? Why doesn’t it get hot? and why does it consume less electricity?
This project is an opportunity to learn more about fluorescent lights and find out how they work.
Adult help and supervision is required for all experiments.
Find out about Fluorescent lights and how they work. Read books, magazines or ask professionals who might know in order to learn about the components and function of an fluorescent light fixture. Keep track of where you got your information from.
Fluorescent light is very different from a regular light (also known as incandescent light). For example in an incandescent light electricity passes through a thin wire known as filament. Electric current will make the filament so hot that it glows and emits light. But in fluorescent lights, electricity passes through a gas (you will learn what type o gas soon) and gas radiates an invisible UV (Ultra Violet) light without getting hot. Then a white powder known as phosphor gets this invisible UV light and produces visible light.
There are some more details on this that you need to learn for your project. For example you need to know about the white powder coated inside the fluorescent light. This white powder that is known as phosphor has nothing to do with the non metal element phosphor. The inner surface of the wall of the tube is coated with light-emitting substances such as zinc sulfide, zinc silicate. The conversion of light from one type to another is called fluorescence, which gave the fluorescent lamp its name. Some minerals are also fluorescent and they will emit visible light under ultra violet lights. You may see pictures of these minerals and see more details about those at http://www.mjt.nu/fluoresce.htm and http://users.rcn.com/kenx/.
If you have a UV flashlight, you can search for fluorescent minerals at your own backyard and display what you find as a part of your project display. Since fluorescent minerals produce light under UV, it is easy to find them at night using a good UV light. Larger UV lights are known as black light and can be purchased from hardware stores.
The other thing you need to know is that why fluorescent lights need a ballast and a starter in order to work? Ballast and starter are two additional parts usually mounted in the fixture of a fluorescent light. A diagram of the fluorescent light is shown here.
The design of the fluorescent lamp is really quite simple. This design remains the same whether the form is a straight tube, circular, or convoluted as in compact fixtures. A fluorescent tube is constructed in its most simple form with a filament similar to an incandescent light bulb at either end, and a coating of a fluorescent material on the inside of the glass envelope. The tube itself vacuum evacuated, a small amount of vaporized mercury is added to the tube, and then it is injected with a small quantity of argon gas. (So the gas inside the florescent bulb is a mixture of argon gas and mercury vapor).
When a current is applied to the filaments at both ends of the tube, the filaments become what are called “cathodes”, meaning that they provide an intense source of positively charged electrons. This energizes the argon gas to a ‘plasma state’ which “excites” the metallic mercury. At this point, the flooding of positive electrons cause the electrons in the shell of the mercury atoms to “jump” (move outward) from a neutral or “ground state” and become “excited”. This pushes electrons outward, filling an “empty” orbital ring with an new electron. The atom then releases its excess electron as the atom attempts to return to its neutral state, and through this process the mercury gas becomes “energized”, driving the excess energy off in the form of photons that lie within the ultraviolet range. The external ballast device serves to limit the amount of current which is delivered to the plasma in this process, maintaining a consistent and uniform source of electrical flow to the cathodes.
This same atom which has just released a photon then picks up another from the cathode flow, continuously repeating the process for as long as the cathode is attached to a source of current.
In the case of mercury, this element emits a very strong line at 2537 Angstroms, well into the far ultraviolet range (UVC). By its own properties, this wavelength is dangerous as none of it ever penetrates to the earth, and life is not prepared to deal with these wavelengths of radiation. But this frequency is helpful in two ways. If no phosphors were added to the tube, this lamp would be the kind that is found in UV sterilizing equipment (such as in barber shops for combs and scissors, and in bowling alleys for shoes), as it will kill all living organisms exposed to it after a time. But this is not the case in consumer fluorescent tubes. This spectral line of photons strike a suitably doped (meaning selected impurities are added) calcium halophosphate coating inside the tube itself, which causes two things to happen. First, the coating filters out the dangerous UVC radiation, and then converts the energy to a different spectral range, mostly that of the visible spectrum.
Depending on the blend of doping materials combined with the calcium halophosphate phosphor, the output range of the tube will vary, and thus a fluorescent device can be custom tailored to produce certain ranges or specifications of light output. This is how various manufacturers produce lighting devices which have different output characteristics. While there are several thousand doping materials which have the potential to change the spectral output of a lamp, only a hundred or so produce useable wavelengths. Characteristics and properties of pertinent lighting devices may be found in the Fluorescent Tubes section.
Fluorescent lamp construction consists of a glass tube with the following features:
- filled with an argon or argon-krypton gas and a small amount of mercury
- coated on the inside with phosphors
- equipped with an electrode at both ends
Fluorescent lamps provide light by the following process.
- An electric discharge (current) is maintained between the electrodes through the mercury vapor and inert gas.
- This current excites the mercury atoms, causing them to emit non-visible ultraviolet (UV) radiation.
- This UV radiation is converted into visible light by the phosphors lining the tube.
Discharge lamps (such as fluorescent) require a ballast to provide correct starting voltage and to regulate the operating current after the lamp has started.
How does a fluorescent light bulb work?
A fluorescent lamp consists of a glass tube that is filled with mercury vapor at low pressure. The inside of the tube is coated with a phosphorous substance. Two coiled metal (tungsten) filaments are at each end of the tube. When an electric current flows through the filaments they start to get hot and glow (like a regular light bulb). When we apply a voltage between the two filaments and electrons get sucked from one filament to the other. While zipping through the tube electrons crash into mercury atoms, which start to glow and send out ultra violet (UV) light.
UV-light is very, very violet. Actually it is so violet that you can’t see it, but you can get a sunburn from it. So on its own UV-light wouldn’t make a useful lamp, that’s why there is a phosphorous substance in the inside of the glass tube. When UV-light hits the phosphor atoms, they absorb the UV light and send out the white light that illuminates your room. The conversion of light from one type to another is called fluorescence, which gave the fluorescent lamp its name.
Fluorescent lights conserve energy. For the same amount of light they need less power than usual light bulbs. By the way, the funny shaped light bulbs made of bend glass tubes in the supermarket are actually fluorescent lamps. Please don’t play and break fluorescent lamps they contain chemicals that are poisonous. When disposing of a fluorescent lamp, you should call the Recycling Center!
Inside a fluorescent light is low-pressure mercury vapor. When ionized, mercury vapor emits ultraviolet light. Human eyes are not sensitive to ultraviolet light (although human skin is). Therefore the inside of a fluorescent light is coated with a phosphor. A phosphor is a substance that can accept energy in one form (for example, energy from a high-speed electron as in a TV tube) and emit the energy in the form of visible light. In a fluorescent lamp the phosphor accepts the energy of ultraviolet photons and emits visible photons.
The light we see from a fluorescent tube is the light given off by the phosphor coating the inside of the tube (the phosphor fluorescence when energized, hence the name).
More about Fluorescent Lamp
The fluorescent lamp is a gas discharge tube whose output of light is so increased by special means that it can be used for lighting purposes. The inner surface of the wall of the tube is coated with light-emitting substances – usually fluorescent or phosphorescent metallic salts (calcium tungstate, zinc sulfide, zinc silicate). The tube is filled with mercury vapor at extremely low pressure. The electrons ejected from the incandescent collide with the mercury atoms and cause these to emit radiation which consists for the most part of ultraviolet rays, which are invisible. The visible portion of the mercury vapor rays is situated in the green and blue range of the spectrum and gives a pale light. The ultraviolet light strikes the fluorescent substance with which the wall of the tube is coated and causes this substance to emit radiation with a longer wavelength in the visible range of the spectrum – i.e., the coating transforms the invisible rays into visible light. By suitable choice of the fluorescent substance, this light can be given any desired color. The lamp has to be operated with a choke, which prevents a harmful rise in voltage and serves to ignite the lamp. For this purpose a small auxiliary glow lamp provided with a thermal contact is connected in parallel with the main lamp. When the current is switched on, the glow lamp first lights up (the bimetallic thermal contact is now open). This causes the bimetallic strip to warm up and close the contact, with the result that the glow lamp is short-circuited and the cathodes of the main lamp receive the full current that makes them incandescent. The bimetallic strip cools and breaks the contact. Through the agency of the choke this interruption of the circuit produces a voltage surge which is high enough to initiate the discharge in the fluorescent lamp itself. Because it is bypassed by the main lamp, the small auxiliary lamp then ceases to function. The bimetallic strip keeps the contact open. The cathodes of the main lamp are kept glowing at white heat by the impingement of positive mercury ions, and the lamp thus continues to function and emit light in the manner described. The light of a fluorescent lamp is not produced by an incandescent body (such as the filament of an ordinary electric lamp), but is emitted as a result of the excitation of atoms (namely, those of the mercury vapor and the fluorescent coating) and is extremely economical. Because of the large light-emitting area, a fluorescent lamp gives a pleasant light which produces only soft shadows.
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to find answers to some of our questions about fluorescent lights including:
- How does a fluorescent light works?
- Why is fluorescent light bulb cold while incandescent light bulbs get very hot?
- Why are there some white powder inside a fluorescent bulb?
- How come some fluorescent light bulbs produce color light?
- What type of gas is inside a fluorescent light?
We try to gather information or perform experiments to find the answers to the above questions.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
This is a research and display project so you will not need to define variables, however if you need to study certain properties of fluorescent lamps, you may need to define variables. For example if you are studying on the effect of fluorescent light on plant growth, you will need to define variables.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
My initial hypothesis was that the white paint inside the fluorescent bulb is conductive and acts as a filament. So I used a multi-meter that can test conductivity and tested that on a broken piece of florescent bulb. surprisingly that paint actually was just some lose powder and it was not conductive at all. So what I propose here is my second hypothesis. (You can come up with your own testable hypothesis)
My hypothesis s that the fluorescent bulb is filled with a conductive gas and the conductive gas acts as a filament and produces the light. White color and consuming less electricity are related to the physical properties of the gas used in such lights.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
In this experiment, we’ll try to excite the gas inside a fluorescent light bulb with some unconventional methods to see if we can make it to go on and get a light.
- Take the comb and light bulb into a dark closet.
- Take the comb and rub it thoroughly through your hair. If you don’t have any hair, a wool shirt or sweater will work fine.
- Now hold the comb to the metal end of the light bulb while carefully watching the filament in the bulb.
Pretty cool, huh? When you rub the comb through your hair, the friction between your hair and the comb actually causes electrons (a negatively charged subatomic particle. Electrons are found at varying distances from a atom’s nucleus. They make up almost the entire volume of a atom but only account for a small part of the atom’s mass.) to travel from your hair to the comb. Your body (hair) becomes positively charged (because it has more protons than electrons) while the comb becomes negatively charged (it gained electrons from your hair) . The comb, in effect, becomes charged. When you touch the comb to the end of the light bulb, the charged comb discharges into the light bulb causing the bulb to emit small pulses of light.
Parent’s Note. Sometimes, a large number of atoms in an object gain or lose electrons. When such a gain or loss happens, the entire object takes on an electric charge (then they’re called ions . The term static electricity describes situations where objects carry electric charge.
Static electricity occurs, for example, when you rub a balloon on your shirt. The friction between the cloth and the balloon causes electrons to transfer from your shirt to the balloon. The shirt then has an overall positive charge because it has more protons than electrons. The balloon takes on a negative charge because it has extra electrons. The balloon will then stick to the shirt or to another surface, such as a wall.
Static electricity has many uses in homes, businesses, and industries. For example, the copying machines found in most offices are electrostatic copiers. They make duplicates of printed or written material by attracting negatively charged particles of toner (powdered ink) to positively charged paper. Static electricity is also used in air cleaners called electrostatic precipitators. These devices put a positive electric charge on particles of dust, smoke, bacteria, or pollen in the air. Negatively charged collector plates attract the positive particles out of the air.
Fluorescent starter is a timed switch. When voltage is applied to the fluorescent lamp:
- The starter is a closed switch and allows current to flow through the filaments at the ends of the tube.
- The current causes the starter’s contacts to heat up and open, thus interrupting the flow of current. At this time the electric current that was traveling through starter will change the path and go through the fluorescent light tube and tube lights.
- Since the lit fluorescent tube has a low resistance, no current will go through the starter any more.
When you turn on a fluorescent tube, the Starter is a closed switch. The filaments at the ends of the tube are heated by electricity, and they create a cloud of electrons inside the tube. The fluorescent starter is a time delay switch which opens after a second or two. When it opens, the voltage across the tube allows a stream of electrons to flow across the tube and ionize the mercury vapor.
Picture of a starter is on the right.
Without the starter, a steady stream of electrons is never created between the two filaments and the lamp flickers.
Inside the casing of a conventional fluorescent starter there is a small gas discharge lamp.
In this experiment we will see if starter of a fluorescent light can be used as an automatic switch to make a regular light bulb to blink.
Warning: This experiment can only be performed using live electricity with obvious risk of electric shock and death. Only students who are supervised by someone with knowledge and experience of electrical safety may perform this experiment.
- Regular small light bulb (15 watt is O.K.)
- Starter (15 watts or 30 watts)
- Socket or base for the light bulb
- Electric insulated wire
- Plug (Consult an electrician on selecting the plug and wire to make sure they match)
Connect one end of the wire pair to the light socket and the other end to the plug. Screw the light bulb and then insert the plug to an electric outlet to make sure that the light comes on. Unplug the wire and then separate the pair of wires. Cut one of the two wires somewhere in the middle, remove the insulator and connect a starter like the image.
At this time your setup has a few bare spots with no insulation. Warn everyone to stay away while doing this experiment. Unplug the your setup as son as you made your observations.
If you are planning to use this setup as a part of your display, consult an electrician for proper insulation of all connections. Place your setup inside a clear plastic cage to make sure that no one will touch it.
We used a round light bulb for this experiment because we were planning to convert this setup to a well known magic display.
The magic display shows a light bulb that is on with no electricity attached to that. For this display you need an extra screw base that you can get from any broken light bulb.
Place the extra screw on the light bulb and use some glue to secure it. Now this looks like an upside down bulb. The only thing that remains is hiding the base in some sand or beans.
The starter and the base can both be hidden under the sand and the power cord can pass through the bottom of the bowel and enter a hole under the bowel and from there get to an electric outlet.
If your school allows to have a display with electricity on that, you may also want to mount a florescent light setup on a board like the following diagram.
This experiment is very similar to our first experiment. The difference is that instead of using static electricity to excite the gas, we try to use a microwave to do the same. We want to see if the mercury gas can be excited in other ways and produce light.
Get a small round fluorescent light and place it in a microwave. Start the microwave and watch to see if the bulb creates any light.
Note: We think this experiment is safe for microwave, specially if you try it only for a short time. However do it at your own risk.
Materials and Equipment:
List of material can be extracted from the experiment section.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
No calculation is required for this project.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.