Question
A monochromatic light source of power $5\ mW$ emits $8 \times 10^{15}$ photons per second. This light ejects photo electrons from a metal surface. The stopping potential for this set up is $2V$. Calculate the work function of the metal.
✓
Answer
$P = 5 \times 10^{-3}W, n = 8 \times 10^{15}$ photons per secondEnergy of each photon,
$\text{E}=\frac{\text{p}}{\text{n}}=\frac{5\times10^{-3}}{8\times10^{15}}=6.25\times10^{-19}\text{J}=\frac{6.25\times10^{-19}\text{J}}{1.6\times10^{-19}}\text{eV}$
$\text{E}=3.9\text{eV}$
Work function$, W_0 = E - V_0 = (3.9 - 2)\ eV = 1.9\ eV$
$\text{E}=\frac{\text{p}}{\text{n}}=\frac{5\times10^{-3}}{8\times10^{15}}=6.25\times10^{-19}\text{J}=\frac{6.25\times10^{-19}\text{J}}{1.6\times10^{-19}}\text{eV}$
$\text{E}=3.9\text{eV}$
Work function$, W_0 = E - V_0 = (3.9 - 2)\ eV = 1.9\ eV$
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