Candidates performance in this question was poor.  Most of them could neither state the
     similarities nor the differences between photoeletronic emission and thermionic emission.
    The expected answers are:
     (a)     Differences between thermionic and photoelectric emission

Thermionic emission	Photoelectric emission
-   Caused by heat/themalenergy -   Electrons are released at a      particular temperature -    Time rate of emission of       electrons increases with      increase in temperature	Caused by lightelectromagnetic wave energy Electrons are released at threshold frequency  Time rate of emission of electrons increases with/is proportional to increase in intensity

           
      (b)    Similarities between thermionic and photoelectric emission
                 

• both are surface phenomena
• electrons are released in both cases
• ejected/emitted electrons have kinetic energy.