Q77 / 90 · KVPY 2012 (SB/SX) · Physics

A stream of photons having energy $3 \mathrm{eV}$ each impinges on a potassium surface. The work function of potassium is $2.3 \mathrm{eV}$. The emerging photo-electrons are slowed down by a copper plate placed $5 \mathrm{~mm}$ away. If the potential difference between the two metal plates is $1 \mathrm{~V}$, the maximum distance the electrons can move away from the potassium surface before being turned back is-

  • Correct answer
    A
    $3.5 \mathrm{~mm}$
  • B
    $1.5 \mathrm{~mm}$
  • C
    $2.5 \mathrm{~mm}$
  • D
    $5.0 \mathrm{~mm}$

Solution

$\mathrm{K}=3-2.3=0.7 \mathrm{eV}, \mathrm{S}=\frac{\mathrm{K}}{\mathrm{eE}}$ and $\mathrm{E}=\mathrm{V} / \mathrm{d}$

Hence, the correct answer is option: 'A'.

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