Modern Physics and Practical/Experimental Skills
This domain covers quantum and nuclear physics alongside the skills examined in practical work. Expect photons, the photoelectric effect, atomic energy levels, de Broglie wavelength, radioactive decay, mass–energy, and experimental design with uncertainties and graphs.
Core rules
- Photon energy E = hf = hc/λ. Photoelectric equation: hf = φ + Kmax, with threshold frequency f0 = φ/h.
- Atomic spectra arise from electron transitions: ΔE = hf. Line spectra are evidence for discrete energy levels.
- de Broglie wavelength λ = h/p.
- Radioactive decay: N = N0e^(−λt), half-life t½ = ln2/λ. Activity A = λN.
- Mass defect Δm releases energy E = Δmc².
- Practical: record repeated readings, identify variables, use uncertainty Δx, and quote percentage uncertainty (Δx/x) × 100%. Plot the quantity that linearises the relationship.
Worked example
Light of wavelength 400 nm falls on a metal of work function 2.0 eV. Photon energy E = hc/λ = (6.63 × 10⁻³⁴)(3.0 × 10⁸)/(4.00 × 10⁻⁷) = 4.97 × 10⁻¹⁹ J. In eV, 4.97 × 10⁻¹⁹ / 1.60 × 10⁻¹⁹ = 3.11 eV. Thus Kmax = 3.11 − 2.0 = 1.11 eV. The reasoning requires converting photon energy to the same unit as the work function before subtracting.
Traps
- Using wavelength in nm without converting to metres.
- Mixing joules and electronvolts in the photoelectric equation.
- Treating half-life as a fixed amount of substance remaining.
- Quoting too many significant figures for an uncertainty.
- Drawing a best-fit line through every point rather than a trend.