Students begin by investigating and comparing the electrical properties of LEDs and incandescent lamps. Students find that LEDs, unlike incandescent lamps, emit light of a single color that does not change when the applied voltage is increased. Students also find that LEDs, unlike incandescent lamps, will only emit light when connected in a certain polarity within the circuit and only above a threshold that depends upon the color of light emitted. Students then can apply their observations to determine whether a Christmas light is an incandescent lamp or an LED.

Following the initial investigations of the electrical properties of LEDs and incandescent lamps, students compare the spectra of these devices with that of gas lamps using inexpensive hand-held spectroscopes. Students may overcome any difficulties in using the spectroscopes by first using rainbow glasses (paper glasses with diffraction gratings as "lenses"). They then use spectroscopes that have an eV scale⁸ to make measurements of the light energy directly. Thus, students do not need to learn the relationship between wavelength and energy of light at this time.

Students may recognize the spectral lines emitted by each gas lamp and the visible spectrum of the incandescent lamp from previous experiences. This investigation, however, may be their first experience with LEDs' broad spectra.

Students realize that LEDs' spectra is different from that of gas lamps and incandescent lamps. Students also recognize that an LED is made of a solid material, which is more complicated than a gas, because the atoms in a solid are more closely packed together and interact with each other. Therefore to understand the spectra of LEDs, one must first understand the spectra of gases.

Students have already observed that gases emit discrete spectral lines. Therefore, they must emit discrete energies of light. Thus students are introduced to an energy level model of an atom (Fig. 1) where the electron's total energy or energy level is represented by a horizontal line.

Figure 1.

Electrons undergo transitions, i.e., they change energy levels by emitting a photon of light. As per the law of energy conservation:

Figure 2 illustrates what happens when a photon of 2.0 eV (red and orange light) of energy is emitted. Thus, by looking at the energy of emitted photons one can learn what is happening in an atom. This process provides students with the opportunity to build models of the atom.

Figure 2.

The advantage of using the energy level model to represent the atom is that students would only require a qualitative understanding of energy and energy conservation to understand how light sources consisting of either gases or solids emit light. The work of Fischler,⁹ Johnston et al.,³ and Petri and Niedderer¹⁰ have shown that students can understand the energy level model of the atom by utilizing concepts of energy and energy conservation in their reasoning. Thus, by using this model students could be introduced to a few basic quantum ideas and could reinforce their understanding of energy conservation.

In our initial development of Visual Quantum Mechanics — Original, when we asked students to construct an energy level diagram from the spectrum of gas, they often used to incorrectly associate the energies of the spectral lines with the energy levels, rather than the transitions between energy levels. To alleviate this misconception, we created a computer program. Students use the software Gas Lamp Emission Spectroscopy to aid in visualizing an energy level model.^11,12 Students select one of the gas lamps on the computer screen and try to match its spectrum by placing energy levels and constructing transitions resulting in spectral lines (Fig. 3). When students drag the energy levels with attached transitions, they immediately see the corresponding changes in the spectral lines that they can match with the real spectrum. Thus, the program allows students to construct an energy level model that explains the spectrum that they observe.

Figure 3.

The program confronts directly the aforementioned misconception that the energy of a spectral line is related to an energy level. In using the program, students quickly find that a spectrum cannot be produced by simply placing energy levels on the vertical energy scale, and that the energy of a spectral line is the energy difference associated with a transition.

An interesting aspect of this approach is that students often find that no two energy level models are identical! For example, one student may produce a model that looks like a ladder with transitions between each step. Another student's model may have a few energy levels close together at the top with each transition going to an energy level located on the bottom. Both of these models are correct since they each produce the desired spectrum. Following this activity, the instructor could introduce the limitations of this model in that it does not give us a unique description of the energies of the atom. Thus, students can learn about the limitations of scientific models in general and how additional information can sometimes resolve the differences between models.

Students then use Gas Lamp Emission Spectroscopy to match the spectrum of an LED in which the spectral lines are grouped together very closely and appear like a spectral band. They find that their model has two sets of energy levels that are grouped closely together.⁶ Thus, students discover that atoms in an LED must have many energy levels that are extremely close together. Following this discovery, students are introduced to the idea that solids have many closely spaced, interacting atoms. These interactions create groups of very closely spaced energy levels called energy bands. LEDs have two energy bands — the conduction (or excited state) and valence band (or ground state), separated by a gap called the energy band gap. Students move to other programs, LED Spectroscopy and Incandescence Spectroscopy, to investigate how energy bands can explain the spectra of LEDs and incandescent lamps.¹¹

Depending on their physics background, students could then move to a computer program that simulates how two semiconductors are combined to construct an LED and how the LED operates in terms of energy bands. The program plays an important role because it illustrates how energy bands of solids can be used to explain student initial investigations of the electrical and spectral properties of LEDs.