A certain metallic surface is illuminated with monochromatic light of… | NEET 2015 (AIPMT, Cancelled Paper)

Q146NEET 2015 · AIPMT · CANCELLED PAPERMedium

Photoelectric Effect: Einstein's Equation

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

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Answer: (d)

Einstein: $\frac{hc}{\lambda} = \phi + eV_s$ ; write $A = \frac{hc}{\lambda}$ .
$A - \phi = 3eV_0$ and $\frac{A}{2} - \phi = eV_0$
Subtract: $\frac{A}{2} = 2eV_0 \Rightarrow A = 4eV_0,\ \phi = eV_0 = \frac{A}{4}$
$\lambda_0 = \frac{hc}{\phi} = 4\lambda$
So the answer is (d) $4\lambda$ .

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