Connection for ap® courses

In this chapter, the basic principles of quantum mechanics are introduced. Quantum mechanics is the branch of physics needed to deal with submicroscopic objects. Because these objects are smaller than those, such as computers, books, or cars, that we can observe directly with our senses, and so generally must be observed with the aid of instruments, parts of quantum mechanics seem as foreign and bizarre as the effects of relative motion near the speed of light. Yet through experimental results, quantum mechanics has been shown to be valid. Truth is often stranger than fiction.

Quantum theory was developed initially to explain the behavior of electromagnetic energy in certain situations, such as blackbody radiation or the photoelectric effect , which could not be understood in terms of classical electrodynamics (Essential Knowledge 1.D.2). In the quantum model, light is treated as a packet of energy called a photon , which has both the properties of a wave and a particle (Essential Knowledge 6.F.3). The energy of a photon is directly proportional to its frequency.

This new model for light provided the foundation for one of the most important ideas in quantum theory: wave-particle duality. Just as light has properties of both waves and particles, matter also has the properties of waves and particles (Essential Knowledge 1.D.1). This interpretation of matter and energy explained observations at the atomic level that could not be explained by classical mechanics or electromagnetic theory (Enduring Understanding 1.D). The quantum interpretation of energy and matter at the atomic level, most notably the internal structure of atoms, supports Big Idea 1 of the AP Physics Curriculum Framework.

Big Idea 1 is also supported by the correspondence principle . Classical mechanics cannot accurately describe systems at the atomic level, whereas quantum mechanics is able to describe systems at both levels. However, the properties of matter that are described by waves become insignificant at the macroscopic level, so that for large systems of matter, the quantum description closely approaches, or corresponds to , the classical description (Essential Knowledge 6.G.1, Essential Knowledge 6.G.2, Essential Knowledge 6.F.3).

Big Ideas 5 and 6 are supported by the descriptions of energy and momentum transfer at the quantum level. Although quantum mechanics overturned a number of fundamental ideas of classical physics, the most important principles, such as energy conservation and momentum conservation, remained intact (Enduring Understanding 5.B, Enduring Understanding 5.D). Quantum mechanics expands on these principles, so that the particle-like behavior of electromagnetic energy describes momentum transfer, while the wave-like behavior of matter accounts for why electrons produce diffraction patterns when they pass through the atomic lattices of crystals.