What is the threshold wavelength in $nm$ for a metal with a work function of $4.0 \ eV$?

For the photoelectric effect,which of the following statements are true?
$I$ The kinetic energies of the photoelectrons do not depend on the frequency of light.
$II$ The photoelectric effect will always occur for highly intense light.
$III$ The maximum kinetic energy of a photoelectron does not depend upon the intensity of the light.
$IV$ The escaping electron's kinetic energy is larger for a larger frequency.

Cut-off potentials for a metal in the photoelectric effect for light of wavelengths $\lambda_1$,$\lambda_2$,and $\lambda_3$ are found to be $V_1$,$V_2$,and $V_3$ volts. If $V_1$,$V_2$,and $V_3$ are in Arithmetic Progression,then $\lambda_1$,$\lambda_2$,and $\lambda_3$ will be in:

Find the number of electrons emitted per second by a $24 \, W$ source of monochromatic light of wavelength $6600 \, \mathring{A}$, assuming $3 \%$ efficiency for the photoelectric effect (take $h = 6.6 \times 10^{-34} \, J \cdot s$ and $c = 3 \times 10^8 \, m/s$).

When a certain metal surface is illuminated with light of wavelength $\lambda$,the stopping potential is $V$. When the same surface is illuminated by light of wavelength $2\lambda$,the stopping potential is $\frac{V}{3}$. The threshold wavelength for the surface is:

The stopping potential required to reduce the photoelectric current to zero is . . . . . .