If the work function of $Na$ metal is $2.3 \ eV$,in which of the following regions of the $EM$ spectrum does the threshold wavelength lie?

The threshold wavelength for photoelectric emission from a material is $5500\,\mathring A$. Photoelectrons will be emitted when this material is illuminated with monochromatic radiation from a:
$A.$ $75\,W$ infra-red lamp
$B.$ $10\,W$ infra-red lamp
$C.$ $75\,W$ ultra-violet lamp
$D.$ $10\,W$ ultra-violet lamp
Choose the correct answer from the options given below:

The threshold frequency for a certain metal is $v_0$. When radiation of frequency $2v_0$ is incident on this metal surface,the maximum velocity of the emitted photoelectrons is $2 \times 10^6 \ m/s$. If radiation of frequency $3v_0$ is incident on the same metal surface,the maximum velocity of the emitted photoelectrons is . . . . . .

Two incident radiations having energies two times and ten times of the work function of a metal surface,produce photoelectric effect. The ratio of maximum velocities of emitted photoelectrons respectively is

In a photo-emissive cell,with exciting wavelength $\lambda$,the maximum kinetic energy of the electron is $K$. If the exciting wavelength is changed to $\frac{3\lambda}{4}$,the kinetic energy of the fastest emitted electron will be:

Photoelectric emission is observed from a metallic surface for frequencies $v_1$ and $v_2$ of the incident light rays $(v_1 > v_2)$. 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