Question

A mercury lamp is a convenient source for studying frequency dependence of photoelectric emission, since, it gives a number of spectral lines ranging from the UV to the red end of the visible spectrum. In our experiment with rubidium photo-cell, the following lines from a mercury source were used
${\lambda }_1=3650\mathring{A},{\lambda }_2=4358\mathring{A},{\lambda }_3=4358\mathring{A},{\lambda }_4=5461\mathring{A},{\lambda }_5=6907\mathring{A}$.

The stopping voltages, respectively, were measured to be:

$V_{01}=1.28V$, $V_{02}=0.95V$, $V_{03}=0.74V$, $V_{04}=0.16V,V_{05}=0V$,

Determine the value of Plancks constant h, the threshold frequency and work function for the material.

Answer 1

Obtain $V_0$ versus plot. The slope of the plot is (h/e) and its intercept on the v-axis is $V_0$. The first four points lie nearly on a straight line which intercepts the v-axis at $V_0=5.0\times {10}^{14}$ Hz (threshold frequency). The fifth point corresponds to $V<V_0$; there is no photoelectrice mission and therefore no stopping voltage is required to stop the current. Slope of the plot is found to be $4.15\times {10}^{15}$ V s. Using $e=1.6\times {10}^{19}C,h=6.64\times {10}^{34}Js$ (standard value $h=6.626\times {10}^{34}Js),{\varphi }_0=h{V}_0=2.11V$.