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Step-by-Step Explanation
1. Understanding the Photoelectric Effect
The photoelectric effect occurs when photons of sufficient energy (or equivalently, of sufficiently small wavelength) strike a material and cause the emission of electrons. The minimum energy needed to liberate electrons from the cathode material is called the work function, and the corresponding wavelength is the threshold wavelength $ \lambda_{0} $.
2. Dependence of Photo-Current on Wavelength
For a given photocell, if light of wavelength $ \lambda $ is incident on the cathode, photoelectrons are emitted only if
$$
\lambda \leq \lambda_{0}.
$$
If $ \lambda $ is larger (i.e., the light is of lower frequency than the threshold frequency), the incident photons lack the energy required to dislodge electrons. Therefore, no photo-electrons are emitted and the photo-current is zero.
3. Effect of Gradually Increasing $ \lambda $
When the cathode is illuminated by decreasing wavelengths (or equivalently, increasing frequency) below $ \lambda_{0} $, photoelectrons will be emitted, and a certain photo-current will be observed at the anode. As $ \lambda $ increases and approaches $ \lambda_{0} $, photo-current continues to flow but may change slightly in magnitude depending on the photon energy. Once $ \lambda $ exceeds $ \lambda_{0} $, the photon energy is insufficient to eject electrons, and the photo-current drops to zero.
4. Conclusion and Correct Option
Therefore, as $ \lambda $ increases beyond the threshold wavelength, the current in the photocell suddenly falls to zero. This behavior matches the graph shown in the correct option:
Hence, this option correctly illustrates that beyond a certain cutoff wavelength, no photoelectrons are emitted and the photocurrent becomes zero.
