Photoelectric Effect
Animated photons knock electrons out of metals — Einstein's KE_max = hf − φ with 4 metal choices.
Key Notes
When light of sufficient frequency strikes a metal surface, electrons are ejected — the photoelectric effect, discovered by Hertz (1887) and explained by Einstein (1905).
Einstein's photon hypothesis: light comes in quanta of energy E = hf. Each photon interacts with ONE electron.
Einstein's equation: K_max = hf − φ, where φ is the work function (minimum energy to release an electron from the metal surface).
Below the threshold frequency f₀ = φ/h, NO electrons are emitted, regardless of intensity. Above, electrons emerge instantaneously.
Maximum KE depends on the frequency, NOT intensity. Intensity affects only the NUMBER of photoelectrons per second.
Stopping potential V_s is the retarding voltage that just stops the most energetic photoelectrons: eV_s = K_max = hf − φ.
Quantum nature: classical wave theory cannot explain (i) threshold frequency, (ii) zero time-delay, (iii) K_max independent of intensity.
Result: light has particle-like (photon) behaviour even though it's a wave — laid the foundation for quantum mechanics.
Formulas
Einstein photoelectric equation
φ = work function (eV). Below f₀ = φ/h, no emission.
Threshold frequency
Minimum frequency for photoemission.
Stopping potential
V_s measured directly in experiments → V_s vs f gives slope h/e.
Photon energy
h = 6.626 × 10⁻³⁴ J·s = 4.136 × 10⁻¹⁵ eV·s.
Photon momentum
Used in radiation pressure and Compton scattering.
Important Points
PHOTON model explains every fact wave theory cannot: threshold, instantaneity, KE vs f, KE independent of I.
Plot of V_s vs f is a straight line; slope = h/e, x-intercept = f₀, y-intercept = −φ/e.
Same metal: brighter (higher intensity) light → MORE electrons but SAME K_max.
Intensity also determines the saturation current — once all emitted electrons reach the anode.
Work function φ depends only on the metal (not on light). Alkali metals (Cs, K, Na) have low φ (~2 eV) → visible light suffices.
Compton effect (X-rays scattered by electrons changing wavelength) is the other key confirmation of photon momentum.
Photoelectric Effect notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.