Modern Physics
5 concepts · ordered by prerequisite depth
Modern physics introduces the ideas that replaced or extended classical physics in the twentieth century. Students study radioactive decay, nuclear fission, and nuclear fusion to understand how unstable nuclei change and how mass can be converted into enormous amounts of energy. They also encounter the photoelectric effect as evidence that light can behave like particles, and special relativity as the theory that explains why time, length, and simultaneity depend on the observer at very high speeds. These topics are often a student's first encounter with physics beyond the classical world of forces and simple motion. They show that the universe at atomic scales and near-light speeds behaves in ways that are deeply surprising but still mathematically precise.
Suggested order: Begin with radioactivity and nuclear processes, then study the photoelectric effect as evidence for quantum ideas, and finish with special relativity and the consequences of the constant speed of light.
Start here
Photoelectric Effect
The photoelectric effect is the emission of electrons from a material when light of high enough frequency shines on it.
Open lesson
Special Relativity
Special relativity is Einstein's theory describing physics at very high speeds, where measurements of time, length, and simultaneity depend on the observer's frame of reference.
Open lesson
Radioactive Decay
Radioactive decay is the spontaneous change of an unstable atomic nucleus into a more stable one, often releasing particles or electromagnetic radiation in the process.
Open lesson