Photoelectric effect is the start of quantum physics. It is very important discovery that light consist of photons (particulate nature). The energy of photon E= hf. Incresing intensity of light does not increase photon energy but only the rate of photons . Only increasing f can increase energy of photon. For photo electrons to be emitted the photon energy must be at least be equal to the work function of the metal. Hence these frequency corresponding to the minimum photon energy is called the threshold frequency.
The stopping potential measures the kinetic energy of the photoelectrons. Increasing the intensity does not increase the stopping potential because the photon energy is unchanged and therefore the ke remains unchanged. Increasing intensity increases the rate of photons hence also the rate of photoelectrons. An increase in photo-current is recorded but not the stopping potential.
