Radically Modern Introductory Physics Text I (Raymond)

The idea for a “radically modern” introductory physics course arose out of frustration with the standard two-semester treatment of the subject. It is basically impossible to incorporate a significant amount of “modern physics” (meaning post-19th century!) in that format. It seemed to us that largely skipping the “interesting stuff” that has transpired since the days of Einstein and Bohr was like teaching biology without any reference to DNA. We felt at the time (and still feel) that an introductory physics course for nonmajors should make an attempt to cover the great accomplishments of physics in the 20th century, since they form such an important part of our scientific culture.

The writing style of the text is quite terse. This partially reflects its origin in a set of lecture notes, but it also focuses the students’ attention on what is really important. Given this structure, a knowledgeable instructor able to offer one-on-one time with students (as in our recitation sections) is essential for student success. The text is most likely to be useful in a sophomore-level course introducing physics majors to the broad world of physics viewed from a modern perspective.

Waves in One Dimension

The wave is a universal phenomenon which occurs in a multitude of physical contexts. The purpose of this section is to describe the kinematics of waves, i. e., to provide tools for describing the form and motion of all waves irrespective of their underlying physical mechanisms.

Many examples of waves are well known to you. You undoubtedly know about ocean waves and have probably played with a stretched slinky toy, producing undulations which move rapidly along the slinky. Other examples of waves are sound, vibrations in solids, and light.

In this chapter we learn first about the basic properties of waves and introduce a special type of wave called the sine wave. Examples of waves seen in the real world are presented. We then learn about the superposition principle, which allows us to construct complex wave patterns by superimposing sine waves. Using these ideas, we discuss the related ideas of beats and interferometry. Finally, the ideas of wave packets and group velocity are introduced.

Transverse and Longitudinal Waves

With the exception of light, waves are undulations in a material medium. For instance, ocean waves are (nearly) vertical undulations in the position of water parcels. The oscillations in neighboring parcels are phased such that a pattern moves across the ocean surface. Waves on a slinky are either transverse , in that the motion of the material of the slinky is perpendicular to the orientation of the slinky, or they are longitudinal , with material motion in the direction of the stretched slinky. (See figure 1.1.) Some media support only longitudinal waves, others support only transverse waves, while yet others support both types. Light waves are purely transverse, while sound waves are purely longitudinal. Ocean waves are a peculiar mixture of transverse and longitudinal, with parcels of water moving in elliptical trajectories as waves pass.

Light is a form of electromagnetic radiation. The undulations in an electromagnetic wave occur in the electric and magnetic fields. These oscillations are perpendicular to the direction of motion of the wave (in a vacuum), which is why we call light a transverse wave.