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  4. The potential difference that must be applied to stop the fastest photo electrons emitted by a nickel surface, having work function 5.01 eV, when ultraviolet light of 200 nm falls on it, must be

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Q. The potential difference that must be applied to stop the fastest photo electrons emitted by a nickel surface, having work function $5.01\, eV$, when ultraviolet light of $200 \,nm$ falls on it, must be

AIPMTAIPMT 2010Dual Nature of Radiation and Matter

Solution:

Here,
Incident wavelength, $\lambda =200 \, nm $
Work function, $\phi _0=5.01 \, eV $
According to Einstein's photoelectric equation
$eV_s=hv-\phi _0$
$eV_s=\frac {hc}{\lambda}-\phi _0$
where $V_s $ is the stopping potential
$eVs= \frac {(1240 \, eV \, nm)}{(200 \, nm)}-5.01 \, eV $
$\, \, \, \, =6.2eV-5.01 \, eV=1.2 \, eV $
Stopping potential, $V_s=1.2 \, V $
The potential difference that must be applied to
stop photoelectrons $=-V_s=-1.2 \, V $