LucidSTEM
A-LEVEL 9702 · A2 · TOPIC 22
Quantum physics
The chain of the topic: light comes in packets of energy E = hf, those photons knock electrons out of metals, the same electrons then diffract like waves, and atoms only hold discrete energy levels so they emit and absorb sharp spectral lines. Around the hexagon are the four ideas; above is what it builds on, below is where it leads.
TOPIC 22: QUANTUM PHYSICS
CAMBRIDGE A-LEVEL PHYSICS 9702 · PATHWAYS
TheLucidSTEM · thelucidstem.com
BUILDS ON
T7 Waves T8 Superposition T11 Particle physics 22.1
22.2
22.3
22.4
TOPIC 22
QUANTUM
PHYSICS
1 · PHOTON: ENERGY & MOMENTUM
Light is delivered in discrete packets.
EM radiation is particulate: a photon is one quantum.
Each photon carries energy set only by frequency.
1 eV = 1.6 × 10−¹⁹ J is the handy energy unit.
E = hf = hc / λ
p = E / c
c
λ
E = hf
one photon, one quantum
2 · PHOTOELECTRIC EFFECT
One photon, one electron, instantly.
No emission below the threshold frequency f₀,
whatever the intensity: energy is not stored up.
Max KE depends on f only; current rises with intensity.
hf = Φ + ½mv²max
Φ = hf₀ f ≥ f₀
metal surface
hf
e−
KE = hf − Φ
3 · WAVE-PARTICLE DUALITY
Particles diffract; waves arrive as quanta.
The photoelectric effect shows light as particles.
Interference and diffraction show light as waves.
Electrons diffracted by a crystal prove matter waves.
Faster particle: larger p, so shorter wavelength.
λ = h / p = h / mv
e−
gap
screen pattern
4 · ENERGY LEVELS & LINE SPECTRA
Atoms hold only discrete energies.
Electrons occupy fixed levels; energies are negative.
A jump down emits a photon; a jump up absorbs one.
Fixed gaps give sharp emission and absorption lines.
Bigger gap means higher f and shorter wavelength.
hf = E₂ − E₁
E₁ (ground)
E₂
E₃
hf
photon emitted on the jump down
LEADS TO
T23 Nuclear physics: quantised energy and E = mc² T24 Medical physics: X-ray photons and attenuation T25 Astronomy: spectral lines and redshift of light Each reuses one idea: photon energy E = hf and the discrete levels that fingerprint atoms across the cosmos.
← Builds on IGCSE: The EM spectrum