The “blacklight” emits strong ultraviolet radiation. Do not stare at the light, and minimize the length of time it is switched on. Paper is provided for drawing representations of spectra obtained by eye.
CHE151Lab7_AtomicSpectra.doc Dr . Roderick M. Macrae CHE 151 You will also have the opportunity to make more accurate measurements of spectra using a miniature fiber-optic based spectrometer. Draw the spectra obtained with this instrument and compare the positions of the major lines obtained in this way with the wavelengths you obtain using the optical spectroscope. More lines will usually be observable in the specta obtained with the fiber-optic spectrometer than can be seen with the naked eye. General characteristics of spectra: (A) One way to characterize spectra is as either emission spectra or absorption spectra. Emission spectra are the spectra of objects that are themselves producing light, such as a glowing light bulb filament or a flame. The spectra produced by solutions containing metal ions through which light shines are absorption spectra; the fact that the liquid appears colored implies that some component of the “white” (polychromatic) light has been absorbed by the liquid and only the remaining components pass through. We have already looked at this phenomenon in another laboratory exercise. What colors are being absorbed by a beaker of green food coloring? What colors are being transmitted? Are there any other possible answers to this question? (B) Daylight is (mainly) an example of a continuous spectrum . List two other light sources that exhibit continuous spectra. (Hint: Things that glow when heated.) (C) Atomic spectra are line spectra . If the emission spectrum of an atom corresponds to a transition from a state of higher energy (of the electrons in the atom) to a state of lower energy, each line corresponds to a difference of energy between two states. What does the fact that you see lines (as opposed to a continuum) tell you about the energies available to the electrons in the atom? Are there any limitations on the energy an electron in an atom can have? A. Qualitative Exercises: 1. Daylight. Point your spectroscope towards the window. You should see a rainbowlike pattern of colors. Write down the highest and lowest wavelengths at which you see light (in nm), and convert them into wavenumber values. Also, using the table below, write down the nanometer and wavenumber values corresponding to each of the color bands you associate with the rainbow (red, orange, etc.). Write down the value corresponding to the approximate center of each color region.
CHE151Lab7_AtomicSpectra.doc Dr . Roderick M. Macrae CHE 151 2. Fluorescent light and incandescent light. Point your spectroscope at a fluorescent tube in the laboratory. Can you describe what you see? How does it compare to daylight? Are there brighter and darker areas? List approximately where the bright bands occur, giving band center values in nanometers and wavenumber units. How does this spectrum compare with
You've reached the end of your free preview.
Want to read all 5 pages?
- Spring '14