The work function of a substance is $4.0 \mathrm{eV}$. Then longest wavelength of light that can cause…

The work function of a substance is $4.0 \mathrm{eV}$. Then longest wavelength of light that can cause photoelectron emission from this substance approximately
  1. $540 \mathrm{~nm}$
  2. $400 \mathrm{~nm}$
  3. $310 \mathrm{~nm}$
  4. $220 \mathrm{~nm}$

Solution

$\frac{\mathrm{hc}}{\lambda}=\mathrm{W}$ $\lambda_{\text {longest }}=\frac{\mathrm{hc}}{\mathrm{W}}=\frac{6.6 \times 10^{-34} \times 3 \times 10^8}{4.0 \times 1.6 \times 10^{-19}}$ $\Rightarrow \lambda_{\text {longest }} \approx 310 \mathrm{~nm}$.

Asked in: JEE Main 2004

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