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 Å, lambda_(2) = 4047 Å, lambda_(3) = 4358 Å, lambda_(4) = 5461 Å, lambda_(5) = 6907 Å`,
The stopping voltages, respectively, were measured to be:
`V_(01) = 1.28 V, V_(02) = 0.95 V, V_(03) = 0.74 V, V_(04) = 0.16 V, V_(05) = 0 V`
Determine the value of Planck’s constant h, the threshold frequency and work function for the material.
[Note: You will notice that to get h from the data, you will need to know e (which you can take to be `1.6 xx 10^(-19) C`). Experiments of this kind on Na, Li, K, etc. were performed by Millikan, who, using his own value of e (from the oil-drop experiment) confirmed Einstein’s photoelectric equation and at the same time gave an independent estimate of the value of h.]

Explanation

Obtain `V_(0)` versus ν plot. The slope of the plot is (h/e) and its intercept on the ν-axis is `ν_(0)` . The first four points lie nearly on a straight line which intercepts the ν-axis at `ν_(0) = 5.0 xx 10^(14) Hz` (threshold frequency). The fifth point corresponds to `v lt v_(0)` , there is no photoelectric emission and therefore no stopping voltage is required to stop the current. Slope of the plot is found to be `4.15 xx10^(-15) v s`. Using `e=1.6xx10^(-19)C, h=6.64xx10^(-34)Js("Standard value h "=6.626xx10^(-34)Js), phi_(0)=hv_(0)=2.11V.`