The work functions for sodium and copper are $2 \ eV$ and $4 \ eV$, respectively. Which of them is suitable for a photocell using $4000 \ \mathring{A}$ light?

The work function of caesium is $2.14 \ eV$. Find
$(a)$ the threshold frequency for caesium,and
$(b)$ the wavelength of the incident light if the photocurrent is brought to zero by a stopping potential of $0.60 \ V$.

Photoelectrons are emitted with maximum velocity $v$ when light of frequency $3f$ is incident on a photosensitive material of work function $2hf$. If the frequency of the incident light is $4.25f$,the maximum velocity of the emitted photoelectrons is ($h$ = Planck's constant).

Photoelectric emission is observed from a metallic surface for frequencies ${\nu _1}$ and ${\nu _2}$ of the incident light rays $({\nu _1} > {\nu _2})$. If the maximum values of kinetic energy of the photoelectrons emitted in the two cases are in the ratio of $1:k$,then the threshold frequency of the metallic surface is

Assertion : In the process of photoelectric emission, all emitted electrons do not have the same kinetic energy.
Reason : If radiation falling on the photosensitive surface of a metal consists of different wavelengths, then the energy acquired by electrons absorbing photons of different wavelengths shall be different.

The retarding potential necessary to stop the emission of photoelectrons,when a target material of work function $1.24 eV$ is irradiated with light of wavelength $4.36 \times 10^{-7} m$ is (in $eV$)