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A certain metallic surface is illuminated with monochromatic light of wavelength λ. The stopping potential for photoelectric current for this light is 3V₀. If the same surface is illuminated with light of wavelength 2λ, the stopping potential is V₀. The threshold wavelength for this surface for photo-electric effect is

1. 4λ
2. λ/4
3. λ/6
4. 6λ

Correct answer: 4λ

Solution

Using the photoelectric equation, we know that stopping potential is related to the energy of the incident light and the work function. For wavelength λ, the stopping potential is 3V₀, and for 2λ, it is V₀. Solving these equations gives the threshold wavelength as 4λ.

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