Modern physics

The big picture

Photons, atoms and nuclei obey the same energy bookkeeping, but the allowed states and physical scales differ. Identify what supplies energy, what stores it and what is measured before substituting values into an equation.

The route connects photon experiments to atomic spectra, then turns to nuclear changes and electronic devices. Compare mechanisms deliberately: an X-ray tube accelerates electrons to make photons, while the photoelectric effect uses photons to release electrons.

What to read, and why

Start at the first stage if the subject is new. Otherwise, use the descriptions to find the missing step in your understanding. All chapter explanations are open; the labels identify chapters whose solutions and answer checks are also free.

1. STAGE 1

   Connect photons and atoms

   Use energy differences and thresholds, then distinguish the continuum cutoff from characteristic X-ray lines.

   Photons, photoelectric emission and matter waves →Separate energy per event from event rate, then connect particle momentum to wavelength.10 worked examples · 8 checks · Complete free chapterHydrogen levels, spectra and ionization →Read negative bound energies correctly and distinguish a level gap, a threshold and a cascade.6 worked examples · 7 checks · Open explanation · member practiceX-ray spectra: cutoffs, characteristic lines and crystal diffraction →Follow energy from an accelerated electron to a photon, then use the spectrum to distinguish the source from the crystal.8 worked examples · 8 checks · Open explanation · member practice

2. STAGE 2

   Follow nuclear change

   Separate the probability of decay from energy released in a reaction.

   Radioactive decay, half-life and mean life →Connect survival probability to measured activity, then test the assumptions behind an inferred age.9 worked examples · 9 checks · Open explanation · member practiceNuclear size, binding and reaction energy →Balance the particles, match the mass convention and include recoil in the energy account.11 worked examples · 9 checks · Open explanation · member practice

3. STAGE 3

   Build electronic decisions

   Use junction behaviour and circuit conditions before reading a logic output.

   Semiconductor junctions, rectifiers and Zener regulation →Explain the carriers, choose a diode model and verify the operating state before trusting the output.17 worked examples · 15 checks · Open explanation · member practiceLogic gates, complete truth tables and useful circuits →Translate the condition, follow intermediate signals and verify every input combination.8 worked examples · 9 checks · Open explanation · member practice

Choose a method before an equation

Use the physical conditions to narrow the choices. A formula becomes useful after you know why its model applies.

Try a complete free problem

These problems include the hints, worked reasoning and answer feedback. Attempt one before opening the solution.