Laser Simulation-Visual Light Lab for High School Students
Marcus McAleer
Pullman High School
Pullman, WA
Lucinda Mendenhall
Lewis-Clark State College
Lewiston, ID
Washington State University Mentors
Dr. Gary Cheng
Mechanical and Materials Engineering
Muhammad Daniel Pirzada
Graduate Student
July, 2005
The project herein was supported by the National Science Foundation Grant No. EEC-0338868: Dr. Richard I. Zollars, Principal Investigator and Dr. Donald C. Orlich, co PI. The module was developed by the authors and does not necessarily represent an official endorsement by the National Science Foundation.Summary & Rationale:
The purpose of this teaching module is to give students a hands-on introductory experience involving the science and engineering applications of lasers and light of various frequencies. This will include the topics of light, color, energy, heat, and absorption. The students will investigate how different objects absorb energy at different frequencies and then use their observations to make predictions and draw conclusions. The intended audience of this module is early high school students but this can easily be modified for students ranging from 6th to 12th grade. The module can be modified to meet the needs of either a science class or a math class (Algebra or Statistics). Each of the four lessons can be accomplished in one 60 minute class period, but can easily be extended for longer blocked periods. Students do not need much prior knowledge for this module. Students should know that white light is a combination of all colors of light, however a quick demonstration of refraction of sunlight with a prism will establish this fact. Students also need to know how to draw a line graph.
Introduction To Lasers & Applications:
LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Einstein was the first to predict that you could have a stimulated emission of radiation in 1917. This was based on the then new theory that the energy levels in atoms were not continuous but discrete. Einstein predicted that when an electron dropped from one energy state to a lower one, a photon containing that specific discrete drop in energy could be emitted. If the energy level of the emitted photons was that of visible light (between 400 and 700 nanometers in wavelength), then we should be able to see this emission of photons as light. This emission of photons could be captured and projected as a beam by using parabolic mirrors. Furthermore, since the energy is very specific, light of one specific wavelength and thus of one specific color could be generated (see below). Also, the electron energy levels are dependent upon the element. Thus, using
different elements to construct lasers would produce lasers of different colors. The first laser ever made was in 1960 which was a solid ruby laser. Since then, many other substances have been used to create a variety of laser light. The laser that most people are familiar with is a helium-neon laser that produces red light. The basic construction of a laser is shown below (the pumping system is the method for initially getting the electrons to a higher energy state).
Lasers produce a focused monochromatic beam of light which allows for a myriad of modern applications. Substances will either reflect, absorb or transmit energy at different wavelengths based on its atomic structure. If a substance absorbs energy at the same wavelength that a laser produces, then that laser can be used to generate heat in a very focused area. Thus, lasers are used in many machining processes in which the desired result is to have holes of exacting micro-dimensions in the substance. The type (color & intensity) of laser that is chosen is dependent upon the material being machined. A laser beam can also be used to melt substances and weld materials on a very small scale. Furthermore, a laser beam can be used in the crystallization of materials used in micro-processing. Medical applications include the ablation of certain tissues which include the now common laser eye surgery. Furthermore, non-invasive laser surgery is now being pursued in the removal of internal cancerous tumors. In these applications, a material (nano-particles) is attached to the tumor (via anti-bodies) and then a laser beam is targeted on the tumor. The color of the laser is chosen in such a way that the skin of the person transmits the specific wavelength of the beam, but the nano-particles absorb it and change the energy into heat. This heat then destroys the tumor without much effect upon the surrounding tissue.
Classroom Applications & Module Overview:
Unfortunately, these applications are not easily shown in the classroom. The lasers used in these applications are expensive and powerful and would most likely introduce a safety hazard for students. Common lasers (like laser pointers) simply do not have enough power to generate significant amounts of heat to be a useful tool for these applications. However, the principles involved can still be investigated by simply focusing high intensity color-filtered light. Various objects can then be placed at the focal point and data can be collected. The data will be in the form of simple visual observations and also temperature recordings of these materials subjected to the focused light. The first lesson will demonstrate how focused white light affects only the small region of the material where the light is focused while leaving the remaining parts unaffected. The subsequent lessons will demonstrate the effects that colored light has on various objects.
To create an apparatus that generates focused colored light will require the following:
§ High powered light source
§ Platform for spotlight with hole in center
§ Colored filters
§ Large focusing lens
§ Platform for lens with hole in it.
§ Stacking materials (wood/brick blocks)
§ Method for holding sample at focus point
For our first apparatus (which we called the laser simulator), we used a 6,000,000 candlepower portable halogen spotlight for our light source. This was suspended over a small chair with the seat removed. The chair was elevated by bricks. At a certain distance below the chair, a platform that contained a lens was placed. A hole was cut out of this platform to hold the lens. We used small 2 ½ pound weights (from the PE department) to hold our samples at the focal point. These were chosen because they allowed us to suspend our samples over the hole in the weights and also because we could adjust the height easily by adding or subtracting weights. The heights needed to be adjusted based on the thickness of our samples. The colored filters we used colored transparent clip boards that we purchased from various office supply stores. These could be either placed on the chair and be used also as a support to the spot light, or they could
be placed directly over the lens. We also used small rounded color filters used for stage lighting. We constructed our second laser simulator out of wood so that we didn’t need the chair or the bricks. The distances from the light source to the lens and from the lens to the focal point will depend on the lens that you use and should be determined before you build your simulator.
Laser Simulator #1 Lens & Weights
Laser Simulator #2 Candle Melting
Considerations, Limitations, & Extensions:
Unfortunately, you cannot exactly obtain the atomic absorption spectrum of a substance in this way. The atomic absorption spectrum of a substance is based on the energy levels of its electrons. Using the laser simulator, you can instead determine the macro absorption spectrum of a substance which is also a function of its molecular structure and its specific heat and is revealed by its color. If an object is a certain color, that means that it is reflecting light of that particular wavelength and absorbing light of the opposite color. For example, the opposite of blue is yellow and thus a blue object is absorbing yellow light and reflecting blue light. This module can easily be modified to emphasize the relationships of color, reflection, transmittance and absorption. However, the baseline that absorption in general depends upon the color of light for a substance will be firmly established.
Unless your filters are professional quality, they probably will be flawed in two main ways. First, your different filters may allow different intensities of light to pass through. However, this can be accounted for by using a light sensor and then dividing your temperature data by the light intensity for each filter. In this way you can directly compare the different effects different colors have on your objects. A second problem will be that some of your filters will only block out the opposite of the transmitting color. For example, your blue filter may be only filtering out yellow light and allowing all other wavelengths to pass through. However, it will still look blue with the absence of yellow,
but will be allowing red, orange, green and purple through.
Your light source may also pose a small problem. Our halogen spotlight only lasted 30 minutes on a battery charge and was unable to run from a normal wall socket. Thus we had to buy an adaptor that would allow us to run the spotlight continuously. Furthermore, halogen lights provide light that is not evenly distributed among the colors. Specifically, yellow light is more intense than any other color. Thus, our blue colored materials heated up more than any other color. This is similar to a problem with your filters and can be overcome in the same way by using a light sensor with each filter and calibrating your results.
Another way to construct your laser simulator is to use a prism and mirrors to isolate the specific band of the spectrum that you desire and then run that color through your lens. This will give you only one color going through your lens and will avoid all problems with filters. However, this may be too difficult to actually construct without expensive equipment.
Either way you design your apparatus, you will need ten of them so that each group of 3 can be all working at the same time. Obviously this may be cost prohibitive. If you do not have it in your budget to buy the needed equipment, then you may want to try to write a grant to obtain the money. If all you can obtain is equipment for one station, then you can work groups through the laser simulator over a longer period of time as you teach a whole unit.
Although many of these issues may limit the level of accuracy in measuring the actual absorption spectrum of materials, the fact that the color of light matters when dealing with absorption of energy will be firmly established. To get more accurate spectral absorption data, a spectrometer should be used. In a spectrometer, white light of uniform intensity across the spectrum is separated out into its color bands with a prism. These bands are then focused with mirrors to pass one at a time through a solution. A light intensity reading is taken before the light hits the solution, and another one is taken after the light passes through the solution. Comparing these two intensities yields the percentage of light that was absorbed by the solution. However, the drawback of using a spectrometer is that all of the action takes place inside the device – out of sight of the students. Students simply put in a solution, press some buttons and then read the numbers. This type of “magic box” may impede the intuitive sense that students should develop regarding light absorption. The laser simulator, on the other hand, is extremely interactive, and although it may provide modest results, it is a great aid in developing student understanding of light, color, absorption, and laser applications. After this module is completed, a follow up lab with a spectrometer would be ideal and would supply more accurate data for further analysis.
Acknowledgements:
We would like to thank Dr. J. Michael Winey (WSU) for enlightening conversations regarding the physics involved in this module. We would also like to thank Mr. Norman Mahler for constructing the 2nd laser simulator for us.
Lab #1-Focused Light
Introduction:
Did you ever play with a magnifying glass outside in the sunlight as a little kid? Today your group will briefly see the effects of focused light. As you proceed, make conjectures about what you see and discuss each others ideas.
Materials needed:
§ Plastic insect
§ Egg white
§ Spatula
§ Small Glass Container
§ Stop Watch
§ Laser Simulator
Safety:
· Never look directly into the light source.
· Do not place your hands at the focal point for more than 1 second when the light source is turned on.
· Wear your safety glasses at all times when the light source is turned on.
· Do not touch the material directly after being removed from the focused light.
· Always be attentive to what you & your partners are doing.
Procedure:
Follow these steps each time an item is placed in the laser simulator.
· Make sure light source is off before placing material under light source.
· Place the object in the dish.
· Place the dish under light source.
· Turn on the light source.
· Quickly adjust the object if needed to make sure it is at the focal point.
· Start timer (if needed)
· Turn off light source at desired time.