KE vs Frequency

Maximum KE of photoelectrons varies LINEARLY with frequency: K_max = hf − φ.

Slope = h (Planck's constant). x-intercept = f₀ = φ/h. y-intercept (at f=0) = −φ.

Below threshold (f < f₀): K_max would be negative — physically means no emission.

Crucially: K_max does NOT depend on intensity. Same f and metal ⇒ same K_max regardless of how bright the light is.

This linearity was a key prediction of Einstein's photon theory — confirmed by Millikan's experiments.

Different metals: parallel lines (same slope h) but different intercepts (different φ).

Photon picture: each photon transfers its full energy hf to ONE electron. The electron uses φ to escape, keeps the rest as KE.