2015 Physics Olympics

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Light

Overview

The WKU Department of Physics & Astronomy is celebrating the International Year of Light by inviting high schools to send teams of four to compete in the 2015 Western Kentucky Physics Olympics. This half-day competition, held Saturday, February 21 in Thompson Center, Central Wing on WKU's Bowling Green campus, consists of a pentathlon of challenging problem-solving activities that reward teamwork, communication and creativity.

The 2015 Physics Olympics competition will commence with two activities that involve competitors arriving at the event ready to compete with devices they have designed, constructed, and tested. The Spectral Scavenger Hunt requires each team to investigate their surroundings to collect evidence of particular specta. For the Cross Polarization event, each team constructs a device with sheets of polarizers that is capable of measuring the specified goals. This year's "Calculation/ Communication Challenge" is titled Spectroscopic Identification and will require each team to divide into pairs for the determination of visual spectra. The "Order-of-Magnitude Quiz" will remain cloaked in secrecy until the day of the event. Laser Targeting, the "On-the-Spot Activity" for this International Year of Light themed competition, will test each team's ability to apply the laws of reflection and refraction in a chaotic, time sensitive situation.

 

Do-Ahead Event: Spectral Scavenger Hunt

The full range of light stretches from the lowest energy, longest wavelength radio waves to the highest energy, shortest wavelength gamma rays. Human eyes are sensitive to a small portion of the electromagnetic spectrum - in the range of wavelengths from about 400 nm to 700 nm (1 nm = 1 billionth of a meter).

A spectroscope (or spectrometer) is an instrument that causes each energy/wavelength to be redirected at a slightly different angle. The resulting spectrum allow for analysis of the amount of light that an object gives off as a function of energy/wavelength. Each physically different source of light has a unique and identifiable spectrum.

Sources of light that appear mostly the same to the naked eye can have completely different physical processes responsible for their emitted illumination. A rainbow is an example of the spectrum of visible sunlight. Modern civilization still uses sunlight as a light source, but we have come to rely more and more on artificial sources of white light illumination such as incandescent bulbs, fluorescent tubes, compact fluorescent bulbs, and light emitting diode (LED) lamps. Spectrometric analysis allows us to determine which physical process is producing the white light emitted by a particular lamp. The spectrum of an incandescent light bulb is very similar to the spectrum of sunlight – indicating that both are created by the same physical process. However, the spectra of a white LED lamp and a fluorescent lamp are different from that of sunlight, as well as from each other. Likewise, any white light (versus a light appearing red or orange-red, yellow-orange, yellow, green, blue, or purple to the naked eye) has observable features in its visible spectrum which indicate how the physical process responsible for its light create a different overall emission that appears different to the naked eye.

A spectroscope can be used to determine if the yellow light in a particular traffic signal is due to yellow LED lamps or to the transmission through a filter placed in front of an incandescent bulb emitting white light. Forensic scientists are able to determine the elemental content of a gas by measuring the wavelengths for which a spectrum produces bright emission. Astrophysicists rely on spectroscopy for most of our understanding of the cosmos. Spectral analysis of the glowing gas in remote galaxies allow us to know the composition of the early universe. The spectrum of a distant star allows us to determine its temperature; with the hottest stars appearing bluish and the coolest stars appearing reddish. Within our solar system, analysis of how each wavelength of the incident sunlight is either reflected or absorbed from a planet or moon is used to understand the material of that world's surface.

 

Spectroscope Construction

Each team that registers by the established deadline will receive a diffraction grating to be used for the dispersive element of its spectroscope. The team is free to use any materials and methods to use this diffraction grating to create a spectroscope. There is not limit to the number of redesigns and rebuilds of the team’s spectroscope; however, the final version must be used to make all submitted observations and this version must be submitted for judging on the day of the competition.

 

Scavenger Hunt Rules

1. Each team will earn points for collecting spectroscopic evidence which clearly indicates that the physical process responsible for the observed emission is different from any of the other light sources submitted by that team.
2. Points are earned for receipt of the required evidence that establishes a particular light source has an emission process different from all other submission from that team.
3. For each observed light source that is claimed by the team to have a physical source for its emission to be different from any other submitted entry, the following evidence is required:
   1. digital image which clearly illustrates how the light source appears to the naked eye;
   2. digital image of one identifiable team member using its final spectroscope to observe the light source;
   3. digital image, obtained through the team’s spectroscope, of the full visible spectrum of the light source;
   4. written description the light source’s location and how it appears to the naked eye;
   5. written description of the spectroscopic features visible on the imaged spectrum that indicate its is created a by a distinct physical process; and
   6. written explanation of the physical process(es) responsible for the imaged spectrum.
4. The team is responsible for locating each light source it observes. The only constraint is that the judges must, in principle, be able to replicate the observation.
5. No points will be awarded for a submission that is not established to have a distinct physical process responsible for its observed visual spectrum.
6. Entries are to be submitted electronically and must be received before 09:00 CST on Friday 20 February 2015.
7. All contestants will ensure that their submissions were obtained through the application of physics principles and generally follow the spirit of the competition.

Scoring

Up to 20 points will be awarded for the construction of a spectroscope using the provided diffraction grating. During Saturday morning's registration period each team must submit the spectroscope used to acquire the submitted spectral observations. Event judges will verify the instrument's ability to measure the features identified in those submissions.

Each submission received by the due date will be judged according to the following criteria.

1. Up to 9 points for receipt of each set of the three required digital images provided as evidence of a spectroscopic observation of the particular light source whose emission is due to a physical process distinct from any other submission by that team. The submitted images must unambiguously support that the team used its spectroscope to observe the particular light source.
2. Up to 2 points for receipt of a written description that could be used by the judges to locate that particular light source and recreate the spectroscopic observation.
3. Up to 2 points for receipt of a written description that effectively and efficiently identifies the spectral features for the particular light source that distinguish it from any of the team's other submissions.
4. Up to 2 points for receipt of a written description that correctly explains how the spectral features observable in the provided digital image of the full visible spectrum determine the physical process(es) responsible for that particular light source.

Overall rankings for this event are determined by greatest total sum of points earned for each submission by that team. Ties will be broken by the points earned for the explanation of the physical processes responsible for each observation.

 

Communication/Calculation Challenge: Spectrum Identification

Your team will be asked to divide into two groups. Two of you will be given spectroscopes (you are also allowed to use the spectroscope your team constructed for the Spectral Scavenger Hunt event), a pencil, and some blank graph paper, with the task of sketching the key features of the spectrum from each of the samples provided by the judges. The other two members will be provided with basic information about the processes responsible for discrete emission spectra, and have the chance to explore the differences in the spectrum from various elements. They will be presented with the first pair’s set of sketches and must correctly identify each source solely from these sketched spectra.

 

Impromptu Team Activity: Laser Targeting

Activity is the key word for this competition, with the goal being for each team to achieve the desired results as quickly as possible. Following the International Year of Light theme, this event will test each team’s ability to apply the laws of reflection and refraction in a chaotic, time sensitive situation. The competition is designed to reward teamwork and common sense thinking as well as knowledge of physics. Every team will come away with smiles and good memories regardless of how well they master the particular challenge.

 

Order-of-Magnitude Quiz (also known as Fermi Questions)

Arrive at a reasonable approximation for the value of a complex situation with very little to no information available to directly compute the answer. In this quiz, the contestants will need to quickly make assumptions for values to use in simple calculations in order to arrive at the "correct" answer, stated as the power of ten of the number that fits the accepted value.

Teams will receive 9 questions to complete within 15 minutes. The teams can divide the work in any way they see fit, but only one answer per question per team will be accepted. Answers will be judged according to how many orders of magnitude the team's answer is from the judge's solution. The lowest score wins -- 0 points awarded for the answer accepted by the panel of judges, with 1 point scored per order of magnitude from the accepted value.

Examples of Order-of-Magnitude Quiz questions include:

• How many electrons enter the starter motor when a new, full-sized pickup starts?
• How many times would a tire of a Ford Taurus rotate when driven from NYC to LA?
• Estimate the number of gallons of gasoline used annually by all the cars in the USA.

 

Plan-Ahead Event: Crossed Polarization

The year's Plan-Ahead competition requires teams to measure the polarization of light for a given situation and use it to correctly determine the rotation of polarization internal to a material.

 

Polarization is a Key Aspect of Light

What is a light beam? A physicist's definition is that light consists of time dependent transverse electric and magnetic waves propagating past a point in space. The oscillations of the electric and magnetic field are always synchronized to be perpendicular to each other and both perpendicular to the direction of travel for the wave. The polarization of the light is defined by the direction of the electric field component of the wave.

Unpolarized light passing through a polarizing filter will have its intensity reduced by a factor of ½. This occurs because only half the originally unpolarized light gets transmitted by the polarizer -- the half with polarization in the same direction as the alignment of the polarizer. Place another polarizer in the light beam, this time with the alignment of its polarizing filter at 90o to the first polarizer -- no light is transmitted through two polarizing filters that are oriented orthogonally. Learn more about polarization at websites such as arborsci.com/cool/polarization or lhup.edu/~dsimanek/14/polaroid.htm

Modern three-dimensional movies project the footage from one camera in polarized light and use orthogonal polarization when projecting the footage from the other camera, the one that captured another perspective. The audience members wear polarizing glasses, with the axis of polarization for one eye orthogonal to that of the other eye.

Your team will use the principle of cross polarization to analyze materials with intrinsic polarizing properties. Some materials have atomic or molecular properties such that light is at least partially polarized when light passes through that substance. When the plane of oscillation of linearly polarized light is observed to be rotated after passing through materials with an asymmetry of their atomic or molecular structure.

 

Patterns of Stress in Transparent Plastics

Some materials will change the polarization of transmitted light when a stress or strain is applied. Placing such materials between two polarizers allows the amount of stress to be measured. Many transparent plastics objects that were formed by casting or with a pressing process have the property of rotating the polarization of transmitted light. The alignment of molecules is changed when under stress. The light passing through the plastic does not change in a way that is easily discernable to the naked eye; however, when the object is placed in a beam of polarized white light and viewed through another polarizer, the internal non-uniformities give rise to colors and patterns.

The effect that makes these colors appear is that the material has the ability to rotate the plane of polarization of the incident light beam. The degree to which the plane of polarization is rotated depends on the color of the light and the stress or characteristic of the material at any given point. The method is an important tool for determining critical stress points in a material.

 

Sugar Concentration

If we polarize white light and pass it through sugar syrup, the direction of polarization of the light emerging from the syrup will be different for the different color components. If the light then passes through a second polarizer, its color changes with the orientation of the transmission axis of this polarizer. A sugar solution rotates the direction of polarization, extinction does not occur when the polarizers are crossed. Determine how much rotation is required to get maximum extinction. You should also note whether you need to give a clockwise or counter-clockwise rotation to the analyzing polarizer. To verify that the rotation is, say 20o clockwise rather than 160o counter-clockwise, you might vary the thickness of the solution. The rotation is due to an asymmetry in the corn syrup molecules. Fruit sugar and turpentine rotate in the opposite direction from sucrose.

 

Instrument Construction

Test your polarimeter with the provided clear plastic fork. The multiple colors are the result of different wavelengths of light. When you pull the tines of the fork apart, or squeeze them together, you will notice that the patterns change.

Use your polarimeter to observe the light transmitted through the provided square glass bottle. Compare what you observe to light passing through an empty bottle to the same bottle with tap water. Now pour some corn syrup into the bottle of water to create a dilute solution and compare what you observe to the previous situations.

 

Rules for the Saturday morning competition

1. Before the competition on Saturday morning, each team will construct a polarimeter and test it in enough different situations to be comfortable obtaining quantitative measurements with it.
2. At the start of Saturday's contest, teams will be given the rules and materials for a particular task requiring quantitative measurements of the rotation of the polarization within a solid or liquid sample.
3. Points will be awarded for the ability to obtain the most accurate results for each sample.

 

Scoring

1. Points will be awarded for the ability to obtain the most accurate results for each sample.
2. Rankings will be determined by the greatest sum of points earned.
3. Ties will initially be broken in favor of the earliest time to submit the results for all samples.
4. All aspects of the team's effort must seek to earn points through application of the principles of physics, and generally follow the spirit of the competition. in the USA.