Modern Physics
Core concepts
Understand the photon model (E = hf = hc/λ), the photoelectric effect (threshold frequency, work function, maximum kinetic energy KE_max = hf − φ), and the de Broglie wavelength λ = h/p. Know atomic energy levels and that emitted or absorbed photon energy equals the level difference. For nuclear physics, interpret mass defect and binding energy via E = mc², and apply conservation of mass number and charge in decay equations. Recognize the basic ideas of special relativity: the invariance of c, time dilation, and length contraction.
Common traps
- Using intensity to explain photoelectric emission; only frequency determines whether electrons are emitted, intensity affects the count.
- Forgetting that KE_max depends on frequency, not intensity.
- Mixing up alpha, beta, and gamma decay products and their effect on atomic number.
- Using classical kinetic energy for photons.
Worked example
A metal has work function 2.0 eV. Light of frequency giving photon energy 3.5 eV strikes it. Then KE_max = 3.5 − 2.0 = 1.5 eV. If the photon energy were below 2.0 eV, no electrons would be emitted regardless of intensity.
Rule of thumb
For photon problems, convert between eV and joules (1 eV = 1.6×10⁻¹⁹ J) and remember E = hc/λ with hc ≈ 1240 eV·nm for quick estimates.