Electromagnetic Spectrum and Doppler Shift
Topic/Concept: Electromagnetic Spectrum/Doppler Shifts
Prerequisite knowledge required: High-school algebra; ability to manipulate simple equations
Resources required:
- Blackboard and chalk
- Electromagnetic spectrum handouts
- Light sources and diffraction gratings
- Computer and projector to show animation of Doppler shift
Learning Goals:
- Realize that electromagnetic radiation is our main pathway to understanding the larger Universe.
- Appreciate that the light they see with their eyes is fundamentally the same phenomenon as all other types of EM radiation, the radio waves they listen to music with, for example.
- Understand the relationships between frequency, wavelength, energy, and temperature.
- Use spectra to analyze the composition and velocities of astronomical objects.
Learning Objectives
- Expose the wave nature of light.
- Use simple mathematical formulae to convert between frequency, wavelength and energy.
- Identify emission/absorption lines in astronomical spectra.
- Apply the Doppler shift formula (∆v/c = ∆λ/λ) to calculate redshift/blueshift in astronomical spectra.
- Find the temperature of a blackbody spectrum using Wien’s Law.
Opening Activity After introducing the topic by means of going over the EM spectrum handout, instructor and students will examine various light sources exhibiting continuum, emission and absorption line spectra of various peak colors. Students will be asked questions about what they see. Which source do you think is hottest? What might account for the differences in spectra of two objects that appear to be the same color?
ExamplesInstructor will introduce and demonstrate the various relationships between frequency, wavelength, energy and temperature and perform example calculations similar to the homework problems.
Students will examine spectra of stars at different temperatures.
Concept Activity/Task: Students will be divided into groups of 4–5 and given paper copies of astrophysical spectra with well-defined emission and absorption lines. The lines will be labeled with their rest wavelengths. In their groups, students will measure the actual positions of the lines on the plots and use their measurements to calculate the velocities of the medium that produces the lines. At the end of the activity, the instructor will reveal the nature of the sources of the spectra.
Assessment: During the concept activity, the instructor makes the rounds of the groups and answers any questions that may come up. Students all put their names on the group work and hand it in at the end of class.
Assignment
Textbook reading. Homework problems based on calculations of wavelength, frequency, and energy of EM radiation (including identification of type of radiation, e.g. radio vs. X-ray based on these quantities), Wien’s Law, and Doppler shift.