What is stopping potential in photoelectric effect

Stopping potential (V₀) is the minimum reverse (retarding) voltage applied between the phototube’s anode and cathode that just reduces the photoelectric current to zero. Physically, it measures the maximum kinetic energy of emitted photoelectrons:

eV₀ = K_max = hν − φ,

where e is electron charge, hν is photon energy, and φ is the metal’s work function. Stopping potential depends on frequency (not intensity): higher frequency → larger V₀; at or below threshold frequency, V₀ → 0 because no emission occurs. 

In exams, define it, give the Einstein equation, and note its dependence on frequency. Typical graph questions plot V₀ vs frequency with slope h/e.