Modern Physics: quanta, atoms and matter waves
Modern Physics: quanta, atoms and matter waves
NEB Class 12 Modern Physics focuses on the photoelectric effect, photon energy, Bohr's hydrogen model, atomic spectra and de Broglie waves. Expect calculations of photon energy, stopping potential, energy levels and wavelengths, plus explanations of quantization.
Core rules
- Photon energy E = hf = hc/λ; useful shortcut hc ≈ 1240 eV nm.
- Photoelectric equation: K_max = hf − φ, where φ is the work function; stopping potential V_s satisfies eV_s = K_max.
- Threshold frequency f₀ = φ/h; no emission below f₀ regardless of intensity.
- Bohr model: E_n = −13.6/n² eV for hydrogen; transition energy ΔE = E_final − E_initial; emitted photon λ = hc/ΔE.
- de Broglie wavelength λ = h/p = h/(mv).
Worked example
Light of wavelength 400 nm falls on a metal with work function 2.0 eV. Find the maximum kinetic energy and stopping potential. Photon energy E = 1240/400 eV = 3.1 eV. K_max = E − φ = 3.1 − 2.0 = 1.1 eV. Since eV_s = K_max, V_s = 1.1 V. The stopping potential depends on frequency, not intensity; brighter light gives more electrons but the same K_max.
Traps
Using wavelength in nm directly in SI equations; mixing joules and electron-volts; calling photon energy the kinetic energy; forgetting that Bohr energies are negative; using n = 0 or non-integer n; applying de Broglie wavelengths to a whole macroscopic object without considering its tiny value.