Ref: Spectrum constructor (Foothill College astronomy simulations)
The purpose of this assignment is to recognize the three different types of spectrum, and explain how spectral lines are used to identify uniquely an atom or molecule in a source of light.
Navigate to the Spectrum Constructor simulation linked above. Recognize the parts of the display: the “eyeball view” is what your eye would see when unaided. The “diffraction grating view” is what your eye would see after the light is passed through a raindrop, cut gemstone, or any other agent that separates light into its constituent colors. The “spectrometer view” is a graph of brightness (vertical axis) vs color (horizontal axis).
The diffraction grating view and spectrometer view are aligned vertically by wavelength. Wavelength is measured in nanometers (nm, 10-9 meters).
Part 1: continuous spectrum
1. Begin with the sim in its default state: the visible spectrum is checked on, the speed is 0 km/s, and all other boxes are unchecked. What you see is called a continuous spectrum. Write a few sentences of description of the diffraction grating view; of the spectrometer view. For example, you might say that in the diffraction view all colors are present, and in the spectrometer view you might say that all colors are at the same brightness (same level on the y-axis).
Part 2: emission line spectrum
1. Select Continuous spectrum: None. Select several atoms from within the Emission spectra box. Describe the changes on the diffraction grating view, and the changes on the spectrometer view. Each bright shade of color that appears is called an emission line. Do any two atoms have the identical set of emission lines? Some of these lines are quite famous: the bright red line of hydrogen at 656 nm is the red color we see in interstellar clouds, and the double yellow line of mercury near 580 nm is found in fluorescent bulbs.
Part 3: absorption line spectrum
1. Select Continuous spectrum: Visible. Uncheck all emission lines. Select several atoms from within the Absorption spectra box. Describe the changes on the diffraction grating view, and the changes on the spectrometer view. Each missing shade of color is called an absorption line, because light is being absorbed (removed) from the spectrum. Is the continuous spectrum necessary for us to see absorption lines? Do any two atoms have the identical set of absorption lines?
Part 4: general questions
1. If an atom has an emission line at a given shade of color (e.g., hydrogen at 656 nm), then does that atom have an absorption line at that same shade of color?
2. Use the slider button under the label km/s to change the speed of the glowing object: negative speeds for objects approaching us, and positive speeds for objects recessing from us. Describe changes to each of the continuous spectrum, emission line spectrum, and absorption line spectrum as the speed changes. This change is called the Doppler effect, and is essential for measuring the speeds of objects located at a distance.
3. Suppose you were given a spectrum of a planet. The spectrum has a collection of absorption lines. Describe how you would use this simulation to identify what lines were present (or absent) in the spectrum, and therefore what atoms were present (or absent) on the planet.