Modern Physics and Atomic Structure
What the exam tests
Modern Physics and Atomic Structure covers the photoelectric effect, line spectra, the Bohr model, de Broglie wavelength, radioactivity and nuclear energy. Questions mix calculation with explanation of evidence for quantised energy levels.
Core rules
- A photon carries energy E = hf = hc/λ, with h = 6.6 × 10⁻³⁴ J·s.
- Photoelectric effect: emission occurs only above a threshold frequency, instantly; intensity affects the number of electrons, not their maximum energy.
- Einstein's equation: hf = W + Eₖ(max), where W is the work function.
- A photon is emitted when an electron falls from a higher to a lower level, with hf equal to the energy difference.
- de Broglie: λ = h/mv.
- Radioactive decay is random and spontaneous; activity halves every half-life.
- In nuclear equations, mass number and atomic number are conserved. E = mc² links mass defect to released energy.
Worked example
Light of frequency 8.0 × 10¹⁴ Hz falls on a metal with work function 3.0 × 10⁻¹⁹ J. Find the maximum kinetic energy of the emitted electrons. E = hf = 6.6 × 10⁻³⁴ × 8.0 × 10¹⁴ = 5.28 × 10⁻¹⁹ J. Eₖ = E − W = 5.28 × 10⁻¹⁹ − 3.0 × 10⁻¹⁹ = 2.28 × 10⁻¹⁹ J.
Traps
- Thinking brighter light gives faster electrons.
- Mixing up work function and ionisation energy.
- Using frequency in kHz without converting to Hz.
- Confusing half-life with a fixed decay time for a single nucleus.
- Forgetting that gamma emission changes energy but not the element.