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VITEEE 2010 Paper

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Question : 29 of 120

Marks: +1, -0

Silver has a work function of 4.7eV. When ultraviolet light of wavelength 100 nm is incident upon it, potential of 7.7 V is required to stop photoelectrons from reaching the collector plate. The potential required to stop electrons when light of wavelength 200 nm is incident upon silver is

1.5 V
1.85 V
1.95 V
2.37 V

Solution:

Given :

\lambda = 100\text{ nm} = 1000\;\text{\AA}

Energy corresponding to

1000\;\text{\AA} = \frac{12375}{1000} = 12.375\text{ eV}

Now,

7.7 = 12.375 - \phi_{0}

or

\phi_{0} = 12.375 - 7.7 = 4.675\text{ eV}

In the second case,

Energy corresponding to

2000\;\text{\AA} = \frac{12375}{2000}\text{ eV} = 6.1875\text{ eV}

Now,

4.7 = 6.1875 - \phi'_{0}

or

\phi'_{0} = 6.1875 - 4.7 = 1.4875 \simeq 1.5\text{ V}

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