Modern Physics Ques 75
- A metal surface is illuminated by light of two different wavelengths $248 $ $nm$ and $310 $ $nm$. The maximum speeds of the photoelectrons corresponding to these wavelengths are $u _1$ and $u _2$, respectively. If the ratio $u _1: u _2=2: 1$ and $h c=1240 e V $ $nm$, the work function of the metal is nearly
(2014 Adv.)
(a) $3.7 $ $eV$
(b) $3.2 $ $eV$
(c) $2.8 $ $eV$
(d) $2.5 $ $eV$
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Answer:
Correct Answer: 75.(a)
Solution:
Formula:
- Energy corresponding to $248 $ $nm$ wavelength
$ =\frac{1240}{248} $ $eV=5 $ $ eV $
Energy corresponding to $310 $ $nm$ wavelength
$=\quad \frac{1240}{310} eV=4 $ $eV $
$\frac{KE _1}{KE _2} =\frac{u _1^{2}}{u _2^{2}}=\frac{4}{1}=\frac{5 eV-W}{4 eV-W} $
$\Rightarrow \quad 16-4 W =5-W \Rightarrow 11=3 $ $W $
$\Rightarrow \quad W =\frac{11}{3}=3.67 eV \cong 3.7 $ $eV$
BETA
