Photoelectric Effect Basics

The photoelectric effect, as presented in NCERT, reveals that light behaves as a stream of energy packets called photons, each carrying energy $E=h\nu$ determined solely by the frequency. Emission of an electron requires a single photon to supply at least the work function of the metal, so if the frequency lies below the threshold value ${\nu }_0$, no individual photon has enough energy to liberate an electron. Increasing the intensity merely increases the number of photons arriving per second, not the energy per photon, so a below-threshold beam cannot eject electrons no matter how bright it is. The option about total energy is wrong because energy arrives in discrete quanta, and it is the per-photon energy that matters, not the accumulated total. The option that brightness eventually compensates is wrong because two low-energy photons do not combine to eject one electron in this regime. The option about a time delay is wrong because photoemission is essentially instantaneous, occurring within nanoseconds when the frequency is sufficient. As stated in NCERT Class 12, Chapter 11 (Dual Nature of Radiation and Matter), the existence of a threshold frequency independent of intensity was the key failure of the classical wave picture. A consistency check confirms that the quantum model, tying emission to per-photon energy, explains the observed frequency threshold that the wave theory could not.