Camping Trip: How can we measure the wavelength of light emitted from stars?
Summary
This activity is designed for students to apply their knowledge of mathematics and physics to "real life" situations. Students are presented with the situation that they are on a camping trip and wish to know the wavelength emitted by a star. How can they measure this? They are given a laser to simulate the starlight and use a diffraction grating and a ruler to determine the wavelength of its light. They compare their results with accepted wavelengths given on the electromagnetic spectrum chart in their text.
This laboratory activity could be used to springboard into a discussion on the "red shift" and stellar emissions.
CollapseLearning Goals
This activity is designed for students to apply the diffraction equation and trigonometry to a "real life" situation. Students construct a right triangle and find the angle between the central (m=0) and the first (m=1) maxima. They use this to calculate the wavelength of the light.
Students then compare their calculation of the wavelength of light to the accepted levels given in the electromagnetic spectrum.
Vocabulary Words: Diffraction, Central Maximum, First Maximum. Wavelength.
Students then compare their calculation of the wavelength of light to the accepted levels given in the electromagnetic spectrum.
Vocabulary Words: Diffraction, Central Maximum, First Maximum. Wavelength.
Context for Use
This is a lab that will require 1 period (about 50 minutes) for explanation and data collection and 20 more minutes for calculation. Students should understand:
1. Basic trigonometry: tangent (angle) = opposite / adjacent, sine (angle) = opposite / hypotenuse;
2. Trigonometric approximations for small angles: sine and tan of small angles are approximately equal to the angles themselves.
2. Basic principles of diffraction grating measurements: path length differences m*lambda = d*sin(angle), where d is the distance between successive slits, and the angle is the angle between the grating and the perpendicular drawn to the next successive ray.
3. Geometry of the screen and the diffraction grating:
tan (angle) = y / L, where the angle is between rays emitted from the diffraction slits, y is the distance between successive maxima on a screen, and L is the distance between the grating and the screen.
1. Basic trigonometry: tangent (angle) = opposite / adjacent, sine (angle) = opposite / hypotenuse;
2. Trigonometric approximations for small angles: sine and tan of small angles are approximately equal to the angles themselves.
2. Basic principles of diffraction grating measurements: path length differences m*lambda = d*sin(angle), where d is the distance between successive slits, and the angle is the angle between the grating and the perpendicular drawn to the next successive ray.
3. Geometry of the screen and the diffraction grating:
tan (angle) = y / L, where the angle is between rays emitted from the diffraction slits, y is the distance between successive maxima on a screen, and L is the distance between the grating and the screen.
Description and Teaching Materials
1. Materials: Students are given a laser, a meter stick, and a diffraction grating.
2. Student directives: Students should read the introduction in the student hand out (attached).
3. Data Collection: Students may need assistance to identify the triangle between the source grating, the screen, and the successive maximum. See the attached teacher guide. After they measure the distance L between the grating and the screen, and the distance y between successive maxima, they are done with data collection.
4. Data Analysis: The next task is to determine the distance d between the slits in the grating. Directions are given on the student hand out and teacher guide.
5. Data Analysis, Continued: Students should find the equation mΛ/d = sin(Θ), or mΛ/d = Θ for small angles in their textbooks. If they let m=1 for the first slit, they get Λ/d = Θ. They already have found d and Θ, so they can compute Λ.
6. Conclusion: Students compare their wavelength to those published in their textbooks. The Student Lab Sheet (Microsoft Word 17kB May25 11) Teacher Guide to the Student Lab Sheet (Microsoft Word 20kB May25 11)
2. Student directives: Students should read the introduction in the student hand out (attached).
3. Data Collection: Students may need assistance to identify the triangle between the source grating, the screen, and the successive maximum. See the attached teacher guide. After they measure the distance L between the grating and the screen, and the distance y between successive maxima, they are done with data collection.
4. Data Analysis: The next task is to determine the distance d between the slits in the grating. Directions are given on the student hand out and teacher guide.
5. Data Analysis, Continued: Students should find the equation mΛ/d = sin(Θ), or mΛ/d = Θ for small angles in their textbooks. If they let m=1 for the first slit, they get Λ/d = Θ. They already have found d and Θ, so they can compute Λ.
6. Conclusion: Students compare their wavelength to those published in their textbooks. The Student Lab Sheet (Microsoft Word 17kB May25 11) Teacher Guide to the Student Lab Sheet (Microsoft Word 20kB May25 11)
Teaching Notes and Tips
Safety note: You can get the red lasers from hardware stores for leveling. Only entrust the lasers to very mature groups of students and with approval from the administration; otherwise, use a para or a parent volunteer to operate the lasers, lest they try to shine them around the room.
Assessment
Students should hand in student hand-out for assessment.
Standards
9-12.II.A.8. The student will describe applications of the different wavelengths of the electromagnetic spectrum.