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A metal surface is exposed to monochromatic radiation of wavelength λ. For this light, the stopping potential required to reduce the photoelectric current to zero is 3V0. When the same surface is illuminated with radiation of wavelength 2λ, the stopping potential becomes V0. The threshold wavelength for this metal is

1. 4λ
2. λ/4
3. λ/6
4. 6λ

Correct answer: 4λ

Solution

The stopping potential is related to the energy of the incident photons, which is inversely proportional to their wavelength. Given that the stopping potential for wavelength λ is 3V0 and for 2λ is V0, we can deduce that the threshold wavelength, which corresponds to the minimum energy needed to eject an electron, must be 4λ, as it would require a photon energy equal to the work function of the metal.

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