Photoelectric Example

Source: "Theory and problems of Modern Physcis", Ronald Gautreau,Problem 9.13

Problem: The emitter in a photoelectric tube has a threshold wavelength of 6000 A. Determine the wavelength of the light incident on the tube if the stopping potential for this light is 2.5 V.

Solution: The work function is

$eW_{0}=h\upsilon _{th}={\frac {hc}{\lambda }}={\frac {12.4\times 10^{3eV.A^{0}}}{6000A^{0}}}=2.07eV$

The photoelectric equation then gives

$ev_{s}=h\upsilon -eW_{0}={\frac {hc}{\lambda }}-eW_{0}$

$2.5eV={\frac {12.4\times 10^{3}eV.A^{0}}{\lambda }}-2.07eV\Rightarrow \lambda =2713A^{0}$