Unit 14: Introduction to Modern Physics
The early-1900s experiments that broke classical physics: particle-like light, atomic fingerprints, decaying nuclei, and bending time.
7 topics · 22 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
The Photoelectric Effect
- How it worksHow does a single photon need enough energy on its own to eject an electron, no matter how many dim photons pile up?
- How it worksHow does the energy of one photon, not the total brightness of the light, decide whether an electron gets ejected at all?
- Going furtherWhat happens to the electrons ejected from a metal when the light's frequency is increased above the threshold?
Your turn1 of 2
Find the energy of a single photon with frequency 14 times 1014 Hz, using Planck's constant h = 6.626 times 10−34 J·s.Atomic Spectra and Energy Levels
- How it worksHow does an electron jumping between fixed energy levels release light at one exact wavelength?
- How it worksHow do only certain electron orbits survive around a nucleus as stable, allowed energy levels?
- Going furtherWhat happens to the color of light emitted when an electron falls from a much higher energy level instead of the next one down?
Your turn1 of 2
Burning a sample of sodium produces a distinctive yellow-orange glow, while burning a sample of neon produces a completely different reddish-orange glow, with each element consistently showing its own unique, narrow set of colored lines rather than a smooth, continuous rainbow of every color. Why does every element burn with its own distinct set of colored lines instead of a smooth rainbow?Radioactive Decay
- How it worksHow does an unstable nucleus emit alpha, beta, or gamma radiation to settle into a more stable configuration?
- How it worksHow does a nucleus decay at a rate set only by its own internal structure, regardless of heat or chemical bonding?
- Going furtherWhat happens to an atom's identity on the periodic table after it undergoes alpha decay?
Your turn1 of 2
A chunk of uranium-bearing rock sitting motionless and untouched on a laboratory shelf continuously shoots out radioactive particles from deep within its own atoms, with no outside trigger, heat source, or chemical reaction needed to start that process. Why can a chunk of rock sitting perfectly still on a shelf still be shooting particles out of its own atoms?Half-Life
- How it worksHow does a fixed fraction of a radioactive sample decay away in every equal stretch of time, no matter how much is left?
- Worked exampleFind how much of a 100 gram sample of a radioactive isotope remains after 3 half-lives.
- Common mistakeWhy doesn't a whole radioactive sample finish decaying after exactly one half-life?
- Going furtherWhat happens to the decay rate of a sample as more and more of it decays away over time?
Your turn1 of 2
Find how much of a 560 gram sample of a radioactive isotope remains after 6 half-lives.Nuclear Fission and Fusion
- How it worksHow does a small amount of mass disappearing in a nuclear reaction convert directly into a huge burst of energy?
- How it worksHow do fission and fusion release energy through opposite processes, one splitting a heavy nucleus apart and the other joining light ones together?
- Going furtherWhat happens inside the Sun's core that lets fusion happen despite the enormous repulsion between hydrogen nuclei?
Your turn1 of 2
Splitting a single uranium nucleus in a fission reaction releases roughly a million times more energy than burning a single molecule of gasoline in an ordinary chemical explosion, an enormous difference for a process happening at such a tiny, individual-atom scale. Why does splitting one uranium nucleus release far more energy than any ordinary chemical explosion?Relativity as a Concept
- How it worksHow does the speed of light staying the same for every observer force space and time themselves to bend?
- How it worksHow does a moving clock end up ticking at a genuinely slower rate for an outside observer, not merely appearing to?
- Going furtherWhat happens to the amount of time dilation as a traveler's speed gets closer and closer to the speed of light?
Your turn1 of 2
A spaceship clock reads a proper time of 140 seconds while the ship travels at 0.652 times the speed of light. Find the time measured by a stationary outside observer.Wave-Particle Duality
- How it worksHow does light and matter both show wave behavior and particle behavior depending on how you look for either one?
- How it worksHow does an electron spread out and interfere like a wave in one setup, yet land as one localized particle in another?
- Going furtherWhat happens to the interference pattern in the double-slit experiment when you try to detect which slit each electron passes through?
Your turn1 of 2
Firing individual electrons one at a time through a pair of narrow slits, with long pauses between each single electron, still builds up the same wave-like interference pattern on the detector screen that a continuous beam of electrons would produce, as if each lone electron somehow passed through both slits simultaneously. Why can a single electron fired at two slits build up an interference pattern as if it went through both at once?