Students begin this unit by investigating the physical properties of different light sources, including an incandescent lamp, LEDs, a Lime Light® night-light, Wint-o-green^TM Lifesavers®, light sticks, and phosphorescent and fluorescent objects. They find that, unlike the incandescent lamp, these other devices do not feel appreciably warm. These devices emit light, not through incandescence in which the primary mechanism used for light emission is thermal energy, but through various forms of luminescence in which some other process involving another type of energy is used. Students also observe the spectra of various light sources, including gas lamps, fluorescent lamps, LEDs, and phosphorescent and fluorescent objects.

Students are introduced to the energy level model in a manner similar to that used in "Solids & Light." They are also introduced to a potential energy diagram — a visual graph of electrical potential energy versus distance to represent a model of an atom. Students also use the Energy Band Creator program. Energy Band Creator allows the students to visualize "atoms" of a gas (a few potential energy diagrams relatively far apart), a pure solid (a large number of potential energy diagrams relatively close together), and a solid with impurities (a large number of potential energy diagrams with a few of varying depth) and their effect on energy levels. The Energy Band Creator helps students investigate how the depth, width, and distance between potential energy diagrams affect energy levels (Fig. 4).

Figure 4.

Adding a large number of impurity atoms to a pure solid results in the formation of a third band of energies lying between the conduction and valence bands. This band of energies, called the impurity band or metastable-state band, is characteristic of luminescent solids. After the students are introduced to the concept of the impurity band, they use computer programs in Spectroscopy Lab Suite to create an energy level model to explain the working of a fluorescent lamp, a phosphorescent toothbrush, and an infrared detector card — a device used by repair people to determine whether remote controls for electronic devices function properly.¹¹

Students have observed the presence of both discrete spectral lines and a broad spectral band for a fluorescent lamp. From their previous observations of gas spectra, they are able to recognize the spectral lines as those from a mercury gas lamp. Students know that the fluorescent lamp is a mercury gas lamp with a material coating on the inner walls of the lamp. Students then use the Fluorescence Spectroscopy program to construct a model to explain the broad spectral band (Fig. 5). Students discover that electrons in the solid coating of a fluorescent glass tube make a transition from the ground-state band to the excited-state band by absorbing ultraviolet light emitted by a mercury gas found inside the tube. These electrons then lose a small amount of energy to neighboring atoms in the form of thermal energy and make a transition to the impurity-state band. Electrons then lose energy in the form of visible light and make a transition to the ground-state band. The program allows the student to edit the properties of the fluorescent glass tube so that various white-light and black-light fluorescent lamps may be modeled.

Figure 5.

In fluorescent materials, electrons have energies in the impurity-state band for a relatively short period of time. Then, they emit light as their energy changes to energy in the valence band. As a result, fluorescent materials will only glow while light of sufficient energy shines on them. In phosphorescent objects, electrons remain in the impurity-state band. After a time delay, the electrons emit light as their energy changes. Thus, phosphorescent materials emit light using energy that was absorbed at an earlier time. When all energy is converted to light, the object stops glowing in the dark.

With the program Fluorescence Spectroscopy, students learn electrons of a glow-in-the-dark toothbrush make a transition from the ground-state band to the excited-state band by absorbing visible light from an external source. These excited electrons lose a small amount of energy in the form of thermal energy to nearby atoms to make the transition from the conduction band to the impurity band. These electrons then absorb thermal energy from the surroundings and make a transition back to the conduction band. The electrons lose energy by emitting the visible light characteristic of glow-in-the-dark objects and make a transition to the valence band. The program allows the student to edit the properties of the external light source and the phosphorescent toothbrush to investigate how these variables affect the resulting spectra.

After exploring the properties of an electronic remote-control device and an infrared detector card, students use the IR Detector Card Spectroscopy program to learn that electrons of an infrared detector card absorb energy from visible light and make a transition to the conduction band. Electrons then lose a small amount of energy to neighboring atoms in the form of thermal energy and make a transition to the impurity-state band. Electrons absorb energy from an infrared light source such as a remote control and then make a transition back to the excited-state band. These electrons then lose energy by emitting visible light and make a transition to the valence band.

After being introduced to these various energy band models to explain luminescent phenomena, students can make comparisons between models.