Photons with energy  5eV are incident on a cathode C, on a photoelectric cell. The maximum energy of the emitted photoelectrons is  2eV.  When photons of energy  6eV are incident on  C, no photoelectrons will reach the anode  A if the  potential of A relative to  C is

Solution:

 Given Photon energy= 5eV
The maximum energy= 2eV
From the Einstein’s photo electric equation, we get
$\varphi =k_{\max }-E_{\varphi }$ 
$\therefore \varphi =5-2=3\text{\hspace{0.17em}eV}$
Now, when photons of energy  6eV are incident, no photoelectrons are emitted. This is because a stopping potential is applied.
$\begin{array}{l}\therefore e{V}_0=E_{\varphi }-\varphi =6-3=3\mathrm{eV}\\ V_0=3\mathrm{V}\end{array}$

Hence, a stopping potential of 3V has to be applied.
Now, stopping potential is defined as the negative potential applied to the anode with respective to the cathode. Hence, stopping potential of  A with respective  C is  -3V