Two identical capacitors are arranged as shown. The work function of plate $1$ is $\phi \ eV$. The $emf$ of the battery is $\frac{\phi}{e}$. If the energy of the incident photon is $hv$,then what is the maximum kinetic energy of the $e^-$ reaching plate $2$?

$A$ point source of light emits photons of energy $4.8 eV$ at the rate $10^5$ photons per second. These photons are incident on a photo-sensitive sphere of work function $2.8 eV$ and radius $9 mm$. The sphere is initially neutral and the emitted photoelectrons are instantly swept away. The time after which the photoelectric emission stops is (in $s$)

The figure shows the variation of photocurrent with anode potential for four different radiations. Let $f_a$,$f_b$,$f_c$ and $f_d$ be the frequencies for the curves $a$,$b$,$c$ and $d$ respectively.

In an experiment with the photoelectric effect,the stopping potential:

The energy of a photon with wavelength $\lambda$ is $2 \ eV$. When it strikes a metal surface,the maximum velocity of the emitted photoelectrons is $v$. If the value of $\lambda$ is decreased by $25\%$ and the maximum velocity is doubled,the work function of the metal becomes ...... $eV$.

The photoelectric threshold wavelength for potassium (work function being $2 \ eV$) is ........... $nm$.