Light of frequency $7.21 \times 10^{14} \; Hz$ is incident on a metal surface. Electrons with a maximum speed of $6.0 \times 10^{5} \; m/s$ are ejected from the surface. What is the threshold frequency for photoemission of electrons?

In the experiment of the photoelectric effect, if the frequency of incident light is $v_1$, the maximum kinetic energy $(K.E.)$ of photoelectrons is $K_0$. If the frequency of light is $v_2$, the maximum $K.E.$ of photoelectrons is $2K_0$. Which of the following relations is correct?

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

The maximum velocity of the photoelectrons emitted by a metal surface is $9 \times 10^5 \ m/s$. The value of the ratio of charge $(e)$ to mass $(m)$ of the photoelectron is $1.8 \times 10^{11} \ C/kg$. The value of the stopping potential in volts is:

An image of the sun is formed by a lens of focal length $30 \ cm$ on the metal surface of a photoelectric cell and a photoelectric current $I$ is produced. The lens forming the image is then replaced by another of the same diameter but of focal length $15 \ cm$. The photoelectric current in this case is

In a photoelectric effect experiment,$f$ is the frequency of radiations incident on the metal surface and $I$ is the intensity of the incident radiations. Consider the following statements. Which of the following statements are correct?
$(A)$ If $f$ is increased keeping $I$ and work function constant,then the maximum kinetic energy of the photoelectron increases.
$(B)$ If the distance between the cathode and anode is increased,the stopping potential increases.
$(C)$ If $I$ is increased keeping $f$ and work function constant,then the stopping potential remains the same and the saturation current increases.
$(D)$ If the work function is decreased keeping $f$ and $I$ constant,then the stopping potential increases.