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Question

When radiation of wavelength λ is incident on a metallic surface, the stopping potential is 4.8 V. If the same surface is illuminated with radiation of double the wavelength, then the stopping potential becomes 1.6 V. Then the threshold wavelength for the surface is :

A
2λ
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B
4λ
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C
6λ
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D
8λ
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Solution

The correct option is B 4λ
4.8×e=hcλϕ1)

1.6×e=hc2λϕ2)

From 1 and 2
so, 3(hc2λϕ)=hcλϕ

3hc2λ3ϕ=hcλϕ

hc2λ=2ϕ

hcλT=hc4λ=ϕ

So, threshold wavelength is 4λ.
Hence, the answer is (B).

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