Emission Spectra lab - My eCoach发射光谱实验室我开放.doc

Summary The student investigates basic properties of spectra in the lab using emission tubes and spectroscopes. Background and Theory Astronomy is nearly entirely a study of light. From this single source of information, we can often find the physical conditions, compositions and processes in distant objects. A plot of the brightness of an object versus wavelength is called a spectrum, (pl. spectra), and is observed using a spectrograph. By spreading out the light by wavelength, we can gain insight into whats happening to photons of particular wavelengths (or energies), which in turn tells us whats happening with particular types of atoms. There are three components of a spectrum: continuum emission (or blackbody radiation), emission lines, and absorption lines. Continuum emission is a wide, smooth (continuous!) band of colors like a rainbow. This type of emission is caused by an opaque material which emits radiation because of its temperature. Hotter objects are brighter and bluer than cooler objects. All objects have continuum radiation. (Even you; although in your case, since its in the infrared, we usually call it heat.) An absorption line is characterized by a lack of radiation at specific wavelength. Absorption lines are created by viewing a hot opaque object through a cooler, thin gas. The cool gas in front absorbs some of the continuum emission from the background source, and re-emits it in another direction, or at another frequency. Absorption lines are subtracted from the continuum emission, so that they appear fainter. An emission line is characterized by excessive radiation at specific wavelengths. You can observe emission lines by looking through a spectrometer at an energized gas. They are created by the photons that are released by the “falling” electrons. The important thing to know about absorption and emission lines is that every atom of a particular element (hydrogen, say) will have the same pattern of lines all the time. And the spacing of the lines is the same in both absorption and emission, only emission lines are added to the continuum, while absorption lines are subtracted. In this lab, you will be using spectrometers to look at various elements which have been heated so that they have many emission lines. You will also observe emission from continuum sources, and from combinations of elements. As you look in the narrow end of the box, look for the wavelength scale inside: 4 through 7 denotes a wavelength scale of 4000 through 7000 The calibration is not very accurate, but usually good to within a few hundred . The electricity from the lamp excites the gas in the bulb. An incandescent lamp glows from thermal emission of a hot metal filament.Conversion: 1 Angstrom = 1 x 10-10 meters=100 pm = 0.1 nm. Purpose: To observe the emission spectra of various elements & combinations of elements. To identify an unknown element by its emission spectrum.Materials: spectrometer (spectroscope); lamp; H, He, N, O, Ne, Cl, Ar bulbs; natural light; incandescent light; fluorescent light; unknown “mystery” light; colored pencils; rulerProcedure & Lab Report: Spectra LabNames_Date_Pd_1. Examine each of the light sources in the table on the worksheet (not necessarily in order!) When observing daylight, look out the window at the sky (clouds are best). Never look directly at the sun with your spectroscope! To observe the fluorescent lights, look at the overhead lights in the hallway.2. Draw the emission spectrum you see roughly on the scale. Try to draw a distinction between sharp and fuzzy lines, bright and faint lines, and regions where continuum is bright and faint. 3. Fill in the spaces (below the numbers 4, 4.5, 5, 5.5, etc.) with the color you perceive or no color. DataTable: Emission/Absorption Spectra of Various Light Sources44.555.566.577.5H44.555.566.577.5He44.555.566.577.5N44.555.566.577.5O44.555.566.577.5Ne44.555.566.577.5Cl44.555.566.577.5Ar44.555.566.577.5Fluorescent light44.555.566.577.5sunlight44.555.566.577.5Incandescent light44.555.566.577.5Mystery gasAnalysis & Conclusion:1. Compare the mystery gas spectrum to those on the spectral chart. What is the mystery gas? _2. Our Sun displays an absorption line spectrum. Examine the solar spectrum carefully and locate the dark absorption lines. At what wavelengths do they appear?3. Promotional signs are often made of lit, colored tubing. These are commonly called neon lights. However, you have seen here that excited neon gas emits a distinctive spectrum and appears orange to the eye. How do you suppose a sign which includes blue and white lettering is made? 4. Describe the difference in appearance between absorption and emission lines.5. Now describe the difference in the physical mechanisms responsible for absorption and emission lines. (How do they work?)6. The fluorescent overhead lights are also gas lamps, although a white fluorescent coating has been placed on the glass of the tube. This coating smears out the spectrum and converts some blue light to redder light, in order to more closely simulate natural light. Identify one of the gases in the lamps which is also present in the overhead lights.(Recall that a certain pattern of strong lines is the signature