Modern physics
Photoelectric effect
Einstein's photoelectric equationhf = φ + KEmaxf₀ = φ/h | eVs = KEmax
KEmax against frequency
Slope = h/e for EVERY metal · x-intercept = threshold f₀ · red dot = your current light
Parameters
Light frequency6 ×10¹⁴ Hz
Threshold for Caesium: f₀ = 5.08×10¹⁴ Hz — drop below it and emission stops
Intensity5
Changes how MANY electrons per second — never their energy
Calculated
Photon energy hf2.48 eV
Wavelength λ500 nm
Work function φ2.10 eV
Threshold f₀5.08 ×10¹⁴ Hz
KEmax0.38 eV
Stopping potential0.38 V
Curriculum
WAECNECOIGCSESATJUPEB
📋 Teacher notes
- •The killer observation classical physics could NOT explain: below the threshold frequency f₀ = φ/h, NO electrons are emitted no matter how intense the light. Try it — set red light on zinc and crank the intensity.
- •Light arrives as PHOTONS of energy E = hf. One photon frees at most one electron: hf = φ + KEmax (Einstein, Nobel Prize 1921).
- •Intensity controls the NUMBER of photons → the photocurrent. Frequency controls the ENERGY of each electron. Watch: more intensity = more electrons, not faster ones.
- •The KEmax–f graph is a straight line with slope h/e (the same for every metal!) and x-intercept f₀. Different metals shift the line, never tilt it.
- •Stopping potential Vs: the reverse voltage that just stops the fastest electrons — eVs = KEmax, so Vs in volts equals KEmax in eV.
✏️ Exercises