In this case, the only photons to reach us are the scattered ones corresponding to the absorption in the previous image. (b) On the other hand, if we observe a cloud that is being illuminated by a bright star that is outside our line of sight, we see emission lines. Because some light has been removed, the spectrum is darker there, though it might not be completely black. This is because some of the light is absorbed by the atoms, then re-emitted in random directions as indicated by the multi-colored “photons” (with arrows showing their direction of travel). Figure 2.6: (a) When a bright object like a star is viewed through a cool cloud of gas, we observe gaps in its continuum, as depicted on the left. This energy source could be, for example, a strong electric current (in the case of neon lights) or a nearby star (in the case of emission nebulae). In contrast to a fluorescent bulb or a gas tube, an incandescent lamp emits a purely continuous Planck spectrum.Īs shown in Figure 2.6, an absorption spectrum is created when a continuous source is viewed (head on) through a low-density cloud, and an emission spectrum is created when a source of energy is exciting a low-density cloud. Incidentally, not all neon signs contain the element neon. A “neon” sign can be a good way to achieve this result. Sometimes, you can see the pure discrete or emission-line spectrum coming from a gas tube that has no coating. The extra bright parts of the spectrum are the places where the discrete spectrum from the gas really stands out. But the coating does not convert all of the light. The coating converts some of the light from the gas into a continuous spectrum. The rainbow is produced by a coating on the inside of the bulb. However, if you look carefully, you will notice that certain colors are especially bright. If you view a fluorescent lamp with a diffraction grating (which spreads light out into its various wavelengths similar to the way a prism does), you will see that it is emitting a rainbow, just like an incandescent lamp. Figure 2.5 shows spectra from some everyday sources of light.įigure 2.5: Spectra from: (a) fluorescent light, (b) neon light, (c) incandescent light. We will learn about two kinds of discrete spectra: emission and absorption spectra. In discrete spectra, only a few frequencies are observed. We now turn to non-continuous, or discrete, spectra. Absorption lines are made when light passes through a colder source like gas clouds, and the gas absorbs particular wavelengths of light.”ĭo you agree with any of these students, and if so, whom? Kensi: “Emission lines are created in hot sources, like a fluorescent lamp.José: “I don’t think so, don’t they come from hot sources? Gas clouds are cold.”.Emission lines come from gas clouds, right?” Isóbel: “I’m having a hard time remembering which type of lines are created by which process.Some students are completing an activity on absorption and emission spectra and discussing their answers. EMISSION AND ABSORPTION IN A HYDROGEN ATOM.CREATION OF SPECTRA FROM A GAS CLOUD: DEPENDENCE ON VIEWING ORIENTATION.
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