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NEET 2016 (Phase 1) · Photoelectric Effect: Einstein's Equation

Photoelectric Effect: Einstein's Equation question from NEET 2016 (Phase 1)

Q141NEET 2016 · PHASE 1Medium

Photoelectric Effect: Einstein's Equation

When a metallic surface is illuminated with radiation of wavelength $\lambda$ , the stopping potential is V. If the same surface is illuminated with radiation of wavelength 2 $\lambda$ , the stopping potential is $\frac{V}{4}$ . The threshold wavelength for the metallic surface is

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Answer: (b)

Einstein: $\frac{hc}{\lambda} = \phi + eV_s$ ; write $A = \frac{hc}{\lambda}$ .
$A - \phi = eV$ and $\frac{A}{2} - \phi = \frac{eV}{4}$
$A - \phi = 4\left(\frac{A}{2} - \phi\right) = 2A - 4\phi \Rightarrow \phi = \frac{A}{3}$
$\lambda_0 = \frac{hc}{\phi} = 3\lambda$
So the answer is (b) $3\lambda$ .

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