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Question

The work function of a substance is 4.0 eV. The longest wavelength of light that can cause photoelectron emission from this substance is approximately:


A
540 nm
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B
400 nm
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C
310 nm
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D
220 nm
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Solution

The correct option is C 310 nm
If $$\lambda_0$$ be the threshold wavelength and $$\lambda$$ be the wavelength of incident light, the condition for photoelectron emission is $$\lambda<\lambda_0$$

Thus, threshold wavelength is the longest wavelength for photoelectron emission. 

So, work function: $$W=\dfrac{hc}{\lambda_0}$$

or, $$\lambda_0=\dfrac{hc}{W}=\dfrac{(6.62\times 10^{-34})(3\times 10^8)}{4\times 1.6\times 10^{-19}}=3.10\times 10^{-7} m=310\times 10^{-9} m=310 nm$$

Physics
NCERT
Standard XII

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