In the previous activities we saw how students can be led to explain the observations of light from atoms and LEDs in terms of the energy changes in atoms. From those observations we helped students develop an energy level model of the atom. We were able to justify the conclusion that only certain values of energies are allowed in free atoms and only certain bands of energies are available to electrons in solids.

This energy level model provides some understanding of the physics of an atom. At this point the students have created discrete energies as an empirical fact. They have not yet found a model that can explain why atoms have discrete energy states. For that explanation we must develop a more sophisticated model of matter.

The model that we are seeking uses waves to describe many types of interactions. So, we have students begin by observing how an interference pattern is a result of interacting waves. To complete this experiment use small pieces of transparencies with waves drawn on them and mounted on pins. Place the pins in a piece of cardboard. Move the waves around and find locations of constructive interference for these two waves. Mark these places.

The nature of matter at the atomic scale is probabilistic rather than deterministic. This concept was an integral paradigm shift for the scientific community when the theory of quantum mechanics was formalized in the early 20th century, and a continuing source of confusion for physics students today. In this activity we will use electron diffraction to illustrate the need for a probabilistic model of matter.