Modern Physics is a first course in the physics that replaced the nineteenth-century picture of space, time, light, and matter. It assumes a year of calculus-based introductory physics — classical mechanics, electromagnetism, waves, and thermodynamics — and uses that foundation to build special relativity, the quantum theory of light and matter, the structure of atoms and molecules, and the physics of nuclei and elementary particles. The book is meant to be readable as a primary text in a one-semester modern-physics course, and usable as a structured reference when those topics appear later.
The material is old enough to be settled and new enough to still feel strange. Special relativity and quantum mechanics are more than a century old; they are also the everyday working language of particle detectors, medical imaging, semiconductor devices, and atomic clocks. The aim here is not to survey every application, but to make the central arguments clear enough that a student who has worked through them can recognize the same physics when it reappears in a research paper, a laboratory, or a later course.
Audience and prerequisites¶
The intended reader has completed — or is completing — an introductory calculus-based physics sequence and is comfortable with derivatives, integrals, and elementary differential equations. Linear algebra beyond what usually appears in that sequence is not required; complex numbers appear where quantum mechanics needs them, and are introduced in place. Prior exposure to special relativity or quantum ideas is welcome but not assumed: each part begins where a careful introductory course typically stops.
Instructors will find that the chapters are sized for assignment by numbered section. A typical semester can cover Parts I–III in full and then choose among Parts IV and V according to taste; a more leisurely pace can stop after the hydrogen atom, or press through to the Standard Model. Nothing later in the book silently depends on a chapter that can reasonably be skipped, beyond the dependencies that the text itself flags.
For a compact 10-week pacing guide with suggested section groupings, see Teaching Guide.
Organization¶
The book has five parts.
Part I — Relativity begins with the experimental failures of the ether picture, develops special relativity from Einstein’s postulates, and closes with relativistic momentum, energy, and the invariant mass that makes particle thresholds computable.
Part II — Wave Optics is a bridge. Interference and diffraction are classical, but they are also the language in which the quantum theory of light and matter is stated. Students who have already seen this material in an introductory course can move quickly; those who have not will find the chapters self-contained.
Part III — Quantum Theory takes up the particle properties of light, the wave properties of matter, the Schrödinger equation in one dimension, and the extension to three dimensions and angular momentum. This is the core of the course for most instructors.
Part IV — Atoms and Molecules applies that machinery to hydrogen, to many-electron atoms and the periodic table, and to the covalent bond and molecular spectra.
Part V — Nuclear and Particle Physics treats nuclear structure and radioactivity, fission and fusion, and — briefly — the elementary particles and the Standard Model, ending where the open questions begin.
How the chapters are built¶
Every chapter opens with learning objectives and a short introduction, then proceeds through numbered reading sections (§N.1, §N.2, …) kept roughly equal in length so that an assignment can be stated as a range of sections. Worked examples are embedded in the reading, not quarantined at the end. Each chapter closes with a summary and a problem set; some also include conceptual questions. Solutions for every problem are included in the complete edition of the book and omitted from the student edition.
A handful of pedagogical asides appear where they earn their keep: notes for historical context, tips for problem-solving strategy, warnings for common misconceptions, and margin remarks for notation or units that would otherwise interrupt the sentence. Dropdowns hold optional derivations. None of these is a substitute for the main argument; all of them can be ignored on a first reading.
Interactive simulations — from PhET and from OpenLyceum — are embedded in the web edition wherever a moving picture teaches more than a static one. In the print and Word editions the same figures appear as screenshots, each caption linking to the running simulation online. Every chapter has at least one.
Editions¶
The website is the primary edition. From the same source the book also builds a complete PDF (with worked solutions), a student PDF (exercises without solutions), a Word document, and a standalone offprint of each chapter. Cross-references, equation numbers, and figure numbers agree across all of them: “Section 4.2” and “Figure 7.3” mean the same thing on the screen, on paper, and in a single-chapter handout.
License and sources¶
This book is released under the Creative Commons Attribution–NonCommercial–ShareAlike 4.0 International License. It adapts openly licensed material — principally OpenStax University Physics Volume 3 and Chemistry 2e, together with several texts from Benjamin Crowell’s Light and Matter series and Paul D’Alessandris’s Spiral Modern Physics — rewritten, reorganized, and extended rather than copied. The per-chapter attribution ledger is the file SOURCES.md in the book’s repository. Diagrams and computed figures created for the book share its license; historical photographs and simulations retain the licenses stated in that ledger.
Corrections and suggestions are welcome. An open textbook is a draft that happens to be useful, and it improves when its readers say so.